botan: Fix include issue that prevented parsing public keys

Botan stopped including build.h in ffi.h with 3.4.0, so we have to
add this here explicitly to check for the enabled key types.

src/libstrongswan/plugins/botan/botan_util_keys.c

@@ -29,6 +29,8 @@
 #include "botan_rsa_public_key.h"
 #include "botan_rsa_private_key.h"
 
+#include <botan/build.h>
+
 #include <asn1/asn1.h>
 #include <asn1/oid.h>