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 <[email protected]>
(cherry picked from commit b3a271f)

Co-authored-by: Alexandru Ardelean <[email protected]>

Author: miss-islington

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

@@ -0,0 +1 @@
+Fixed SSL module build with OpenSSL & pedantic CFLAGS.

Modules/_ssl.c

@@ -78,6 +78,7 @@ static PySocketModule_APIObject PySocketModule;
 #include "openssl/err.h"
 #include "openssl/rand.h"
 #include "openssl/bio.h"
+#include "openssl/dh.h"
 
 /* SSL error object */
 static PyObject *PySSLErrorObject;