string: Fix -Werror=discarded-qualifiers with GCC 15

GCC 15 tightened its built-in declarations for strchr() and strstr() so
that they now propagate const from their first argument, triggering
-Werror=discarded-qualifiers on three assignments in string.c:

  translate():
    char *match = strchr(from, *pos)
    `from` is const char *, so the result of strchr() is const char *.
    `match` is only used for pointer arithmetic (match - from), so
    declaring it const char * is correct and safe.

  strreplace():
    found = strstr(str, search)         [line ~73]
    found = strstr(pos, search)         [line ~89, while condition]
    `str`/`pos` are derived from a const char * parameter, so strstr()
    returns const char *. `found` is used as a mutable char * later
    (pos = found + slen), consistent with the existing (char*) casts
    already used throughout this function for the same reason.
    Add explicit (char*) casts to match the established pattern.

Closes strongswan/strongswan#3015

src/libstrongswan/utils/utils/string.c

@@ -29,7 +29,7 @@ char* translate(char *str, const char *from, const char *to)
 	}
 	while (pos && *pos)
 	{
-		char *match;
+		const char *match;
 		if ((match = strchr(from, *pos)) != NULL)
 		{
 			*pos = to[match - from];
@@ -70,7 +70,7 @@ char* strreplace(const char *str, const char *search, const char *replace)
 	{
 		len = strlen(str);
 	}
-	found = strstr(str, search);
+	found = (char*)strstr(str, search);
 	if (!found)
 	{
 		return (char*)str;
@@ -86,7 +86,7 @@ char* strreplace(const char *str, const char *search, const char *replace)
 		dst += rlen;
 		pos = found + slen;
 	}
-	while ((found = strstr(pos, search)));
+	while ((found = (char*)strstr(pos, search)));
 	strcpy(dst, pos);
 	return res;
 }