BUG/MEDIUM: build without openssl broken

The commit 872f9c213 ("MEDIUM: ssl: add basic support for OpenSSL crypto
engine") broke the build without openssl support.

The ssl_free_dh() function is not defined when USE_OPENSSL is not
defined and leads to a compilation failure.
diff --git a/src/haproxy.c b/src/haproxy.c
index 60bd334..9cbb3d5 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2575,7 +2575,7 @@
 				protocol_unbind_all();
 				mworker_wait();
 			}
-#ifndef OPENSSL_NO_DH
+#if defined(USE_OPENSSL) && !defined(OPENSSL_NO_DH)
 			ssl_free_dh();
 #endif
 			/* should never get there */