Fix -Winconsistent-missing-override in SkCodec_libgif.h

Follow up to https://codereview.chromium.org/1400343005
This warning only triggers on a few configs, like the coverage bot.

TBR=msarett@google.com

BUG=skia:

Review URL: https://codereview.chromium.org/1407833002
1 file changed