certificate: Return signature scheme and parameters from issued_by() method

This also required some include restructuring (avoid including library.h
in headers) to avoid unresolvable circular dependencies.

src/libstrongswan/plugins/openssl/openssl_sha1_prf.c

@@ -20,6 +20,7 @@
 #include "openssl_sha1_prf.h"
 
 #include <openssl/sha.h>
+#include <crypto/hashers/hasher.h>
 
 typedef struct private_openssl_sha1_prf_t private_openssl_sha1_prf_t;