Fix -Winconsistent-missing-override in SkCodec_libgif.h

src/codec/SkCodec_libgif.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCodec.h"
 #include "SkColorTable.h"
 #include "SkImageInfo.h"
@@ skipping 52 matching lines @@
             SkPMColor*, int*, int*) override;
 
     SkEncodedFormat onGetEncodedFormat() const override {
         return kGIF_SkEncodedFormat;
     }
 
     bool onRewind() override;
 
     uint32_t onGetFillValue(SkColorType colorType, SkAlphaType alphaType) const override;
 
-    int onOutputScanline(int inputScanline) const;
+    int onOutputScanline(int inputScanline) const override;
 
 private:
 
     /*
      * A gif can contain multiple image frames.  We will only decode the first
      * frame.  This function reads up to the first image frame, processing
      * transparency and/or animation information that comes before the image
      * data.
      *
      * @param gif        Pointer to the library type that manages the gif decode
@@ skipping 98 matching lines @@
     SkAutoTDeleteArray<uint8_t>             fSrcBuffer;
     const SkIRect                           fFrameRect;
     const uint32_t                          fTransIndex;
     uint32_t                                fFillIndex;
     const bool                              fFrameIsSubset;
     SkAutoTDelete<SkSwizzler>               fSwizzler;
     SkAutoTUnref<SkColorTable>              fColorTable;
 
     typedef SkCodec INHERITED;
 };