Add a typedef for the construction function

Reviewed-by: Rich Salz <rsalz@openssl.org>
diff --git a/ssl/statem/statem_clnt.c b/ssl/statem/statem_clnt.c
index 1ea9d75..08b1239 100644
--- a/ssl/statem/statem_clnt.c
+++ b/ssl/statem/statem_clnt.c
@@ -512,8 +512,7 @@
  *   0: Error
  */
 int ossl_statem_client_construct_message(SSL *s, WPACKET *pkt,
-                                         int (**confunc) (SSL *s, WPACKET *pkt),
-                                         int *mt)
+                                         confunc_f *confunc, int *mt)
 {
     OSSL_STATEM *st = &s->statem;