pki_gcrypt: Warn about unsupported PEM export in gcrypt

Based on the following mail thread:

https://www.libssh.org/archive/libssh/2019-12/0000027.html

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/pki_gcrypt.c

@@ -938,6 +938,8 @@ ssh_string pki_private_key_to_pem(const ssh_key key,
     (void) auth_fn;
     (void) auth_data;
 
+    SSH_LOG(SSH_LOG_WARN, "PEM export not supported by gcrypt backend!");
+
     return NULL;
 }