bpo-34710: fix SSL module build (GH-9347)

Include ``openssl/dh.h`` header file to fix implicit function declaration of ``DH_free()``.
Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

Misc/NEWS.d/next/Build/2018-09-17-13-56-12.bpo-34710.ARqIAK.rst

+Fixed SSL module build with OpenSSL & pedantic CFLAGS.

Modules/_ssl.c

@@ -63,6 +63,7 @@ static PySocketModule_APIObject PySocketModule;
 #include "openssl/err.h"
 #include "openssl/rand.h"
 #include "openssl/bio.h"
+#include "openssl/dh.h"
 
 #ifndef HAVE_X509_VERIFY_PARAM_SET1_HOST
 #  ifdef LIBRESSL_VERSION_NUMBER