[2.7] bpo-34710: fix SSL module build (GH-9347) (GH-9353)

Include ``openssl/dh.h`` header file to fix implicit function declaration of ``DH_free()``.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>.
(cherry picked from commit b3a271fc)
Co-authored-by: Alexandru Ardelean <ardeleanalex@gmail.com>



https://bugs.python.org/issue34710

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

@@ -74,6 +74,7 @@
 #include "openssl/ssl.h"
 #include "openssl/err.h"
 #include "openssl/rand.h"
+#include "openssl/dh.h"
 
 /* SSL error object */
 static PyObject *PySSLErrorObject;