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>
diff --git a/Misc/NEWS.d/next/Build/2018-09-17-13-56-12.bpo-34710.ARqIAK.rst b/Misc/NEWS.d/next/Build/2018-09-17-13-56-12.bpo-34710.ARqIAK.rst
new file mode 100644
index 0000000..b06289d
--- /dev/null
+++ b/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.
diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index 2bce481..3c93884 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -63,6 +63,7 @@
 #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