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
diff --git a/src/codec/SkCodec_libgif.h b/src/codec/SkCodec_libgif.h
index 2e0c5a4..31e0139 100644
--- a/src/codec/SkCodec_libgif.h
+++ b/src/codec/SkCodec_libgif.h
@@ -70,7 +70,7 @@
 
     uint32_t onGetFillValue(SkColorType colorType, SkAlphaType alphaType) const override;
 
-    int onOutputScanline(int inputScanline) const;
+    int onOutputScanline(int inputScanline) const override;
 
 private: