Cast uses of return_*(), nop() and enumerator_create_empty()

As described in the previous commit, GCC 15 uses C23 by default and that
changes the meaning of such argument-less function declarations.  So
whenever we assign such a function to a pointer that expects a function
with arguments it causes an incompatible pointer type warning.  We
could define dedicated functions/callbacks whenever necessary, but this
seems like the simpler approach for now (especially since most uses of
these functions have already been cast).

src/charon-nm/nm/nm_handler.c

@@ -195,7 +195,7 @@ nm_handler_t *nm_handler_create()
 		.public = {
 			.handler = {
 				.handle = _handle,
-				.release = nop,
+				.release = (void*)nop,
 				.create_attribute_enumerator = _create_attribute_enumerator,
 			},
 			.create_enumerator = _create_enumerator,

src/libcharon/encoding/payloads/encrypted_payload.c

@@ -1023,7 +1023,7 @@ encrypted_fragment_payload_t *encrypted_fragment_payload_create()
 				.get_length = _frag_get_length,
 				.add_payload = _frag_add_payload,
 				.remove_payload = (void*)return_null,
-				.generate_payloads = nop,
+				.generate_payloads = (void*)nop,
 				.set_transform = _frag_set_transform,
 				.get_transform = _frag_get_transform,
 				.encrypt = _frag_encrypt,

src/libcharon/plugins/android_dns/android_dns_handler.c

@@ -191,7 +191,7 @@ METHOD(enumerator_t, enumerate_dns, bool,
 	VA_ARGS_VGET(args, type, data);
 	*type = INTERNAL_IP4_DNS;
 	*data = chunk_empty;
-	this->venumerate = return_false;
+	this->venumerate = (void*)return_false;
 	return TRUE;
 }
 

src/libcharon/plugins/ha/ha_attribute.c

@@ -381,7 +381,7 @@ ha_attribute_t *ha_attribute_create(ha_kernel_t *kernel, ha_segments_t *segments
 			.provider = {
 				.acquire_address = _acquire_address,
 				.release_address = _release_address,
-				.create_attribute_enumerator = enumerator_create_empty,
+				.create_attribute_enumerator = (void*)enumerator_create_empty,
 			},
 			.reserve = _reserve,
 			.destroy = _destroy,

src/libcharon/plugins/updown/updown_handler.c

@@ -220,7 +220,7 @@ updown_handler_t *updown_handler_create()
 			.handler = {
 				.handle = _handle,
 				.release = _release,
-				.create_attribute_enumerator = enumerator_create_empty,
+				.create_attribute_enumerator = (void*)enumerator_create_empty,
 			},
 			.create_dns_enumerator = _create_dns_enumerator,
 			.destroy = _destroy,

src/libstrongswan/utils/identification.c

@@ -1625,7 +1625,7 @@ static private_identification_t *identification_create(id_type_t type)
 			this->public.hash = _hash_binary;
 			this->public.equals = _equals_binary;
 			this->public.matches = _matches_any;
-			this->public.contains_wildcards = return_true;
+			this->public.contains_wildcards = (void*)return_true;
 			break;
 		case ID_FQDN:
 		case ID_RFC822_ADDR:
@@ -1660,13 +1660,13 @@ static private_identification_t *identification_create(id_type_t type)
 			this->public.hash = _hash_binary;
 			this->public.equals = _equals_binary;
 			this->public.matches = _matches_range;
-			this->public.contains_wildcards = return_false;
+			this->public.contains_wildcards = (void*)return_false;
 			break;
 		default:
 			this->public.hash = _hash_binary;
 			this->public.equals = _equals_binary;
 			this->public.matches = _matches_binary;
-			this->public.contains_wildcards = return_false;
+			this->public.contains_wildcards = (void*)return_false;
 			break;
 	}
 	return this;