Missing "obj_mac" header file in "dh_lib"

Usage of `NID_undef` symbol without including its definition was causing
a build fail

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/10996)

crypto/dh/dh_lib.c

diff --git a/crypto/dh/dh_lib.c b/crypto/dh/dh_lib.c
index f9fb1d9..0c1cccb 100644
--- a/crypto/dh/dh_lib.c
+++ b/crypto/dh/dh_lib.c
@@ -8,13 +8,13 @@
  */
 
 #include <stdio.h>
+#include <openssl/bn.h>
+#include <openssl/engine.h>
+#include <openssl/obj_mac.h>
 #include "internal/cryptlib.h"
 #include "internal/refcount.h"
-#include <openssl/bn.h>
+#include "crypto/dh.h"
 #include "dh_local.h"
-#include "crypto/dh.h"
-#include <openssl/engine.h>
-#include "crypto/dh.h"
 
 #ifndef FIPS_MODE
 int DH_set_method(DH *dh, const DH_METHOD *meth)