Add libgifcodec to third_party/.gitignore

libgifcodec was recently added to the DEPS for gclient sync:
https://chromium-review.googlesource.com/c/chromium/src/+/1944286

Synced files should be ignored by git.

Bug: 1014044, 1030577
Change-Id: I1fbd3c73368e0c13eb4b314f5bf0606af3b39992
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1949810
Auto-Submit: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721358}

third_party/.gitignore

@@ -132,6 +132,7 @@
 /libevdev/src
 /libexif/sources
 /libFuzzer/src
+/libgifcodec
 /libprotobuf-mutator/src
 /libipp/libipp
 /libjingle/source