Add IWYU pragma to obj_mac.h.

This pragma is similar to the one in obj.h, and allows for matching
openssl compatibility with internal tooling.

Change-Id: I42b1f2527f2b9848d830100ec4a0dc0338619e4e
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/76807
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>

include/openssl/obj_mac.h

@@ -15,4 +15,4 @@
 /* This header is provided in order to make compiling against code that expects
    OpenSSL easier. */
 
-#include "nid.h"
+#include "nid.h"  // IWYU pragma: export