Mark SkGifImageReader destructors with override

Very new versions of Clang (5.0 TOT) call out this problem.

Bug: skia:
Change-Id: If90f658f258ca52ebb21e2352fbf3180c41008bd
Reviewed-on: https://skia-review.googlesource.com/17795
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
1 file changed