OpenSSL EVP: fix threaded use of structs

Make sure we don't clear or reset static structs after first init so
that they work fine even when used from multiple threads. Init the
structs in the global init.

Help and assistance by: John Engstrom

Fixes #229 (again)

src/libgcrypt.c

@@ -585,4 +585,8 @@ _libssh2_pub_priv_keyfile(LIBSSH2_SESSION *session,
                   libssh2_error */
 }
 
+void _libssh2_init_aes_ctr(void)
+{
+    /* no implementation */
+}
 #endif /* LIBSSH2_LIBGCRYPT */