Only provide new libpq sslpasskey hook for openssl-enabled builds

In commit 4dc6355210 I neglected to put #ifdef USE_OPENSSL around the
declarations of the new items. This is remedied here.

Per complaint from Daniel Gustafsson.

src/interfaces/libpq/libpq-fe.h

@@ -620,10 +620,12 @@ extern int	pg_valid_server_encoding_id(int encoding);
 /* == in fe-secure-openssl.c === */
 
 /* Support for overriding sslpassword handling with a callback. */
+#ifdef USE_OPENSSL
 typedef int (*PQsslKeyPassHook_type)(char *buf, int size, PGconn *conn);
 extern PQsslKeyPassHook_type PQgetSSLKeyPassHook(void);
 extern void PQsetSSLKeyPassHook(PQsslKeyPassHook_type hook);
 extern int PQdefaultSSLKeyPassHook(char *buf, int size, PGconn *conn);
+#endif
 
 #ifdef __cplusplus
 }