DGifCloseFile signature changed: fixes build against giflib 5.1.0

I just tried to build skippy-xd on arch linux, but it failed (even though it did work 2 days ago).

I'm using this package build in the user repository:
https://aur.archlinux.org/packages/skippy-xd-git/

I'm not sure about the versioning of libgif, but this patch is necessary for skippy-xd to compile and work at least on arch.

Here's the line in the giflib header:
http://sourceforge.net/p/giflib/code/ci/master/tree/lib/gif_lib.h#l183