GIF loader: Fix for libgif version 5.1

Summary:
From giflib-5.1.0's NEWS:
"A small change to the API: DGifClose() and EGifClose() now take a
pointer-to-int second argument (like the corresponding openers)
where a diagnostic code will be deposited when they return
GIF_ERROR."

Test Plan:
I've built imlib2 against giflib-4.2.3 and 5.1.0 and opened a few
gif files with feh.

Reviewers: kwo

Reviewed By: kwo

Differential Revision: https://phab.enlightenment.org/D1529

src/modules/loaders/loader_gif.c

@@ -198,7 +198,11 @@ load(ImlibImage * im, ImlibProgressFunction progress, char progress_granularity,
    free(rows);
 
  quit2:
+#if GIFLIB_MAJOR > 5 || (GIFLIB_MAJOR == 5 && GIFLIB_MINOR >= 1)
+   DGifCloseFile(gif, NULL);
+#else
    DGifCloseFile(gif);
+#endif
 
    return rc;
 }