Put close_gif() in an anonymous namespace.

Not a big deal, but matches the intention of it being a file-scoped function
better.  This mirrors a recent google3 change for clients who don't use C++11.

BUG=skia:

Review URL: https://codereview.chromium.org/865243002
diff --git a/src/images/SkImageDecoder_libgif.cpp b/src/images/SkImageDecoder_libgif.cpp
index fa12301..c86daf3 100644
--- a/src/images/SkImageDecoder_libgif.cpp
+++ b/src/images/SkImageDecoder_libgif.cpp
@@ -229,7 +229,8 @@
     }
 }
 
-int close_gif(GifFileType*);  // This function is a template argument, so can't be static.
+namespace {
+// This function is a template argument, so can't be static.
 int close_gif(GifFileType* gif) {
 #if GIFLIB_MAJOR < 5 || (GIFLIB_MAJOR == 5 && GIFLIB_MINOR == 0)
     return DGifCloseFile(gif);
@@ -237,6 +238,7 @@
     return DGifCloseFile(gif, NULL);
 #endif
 }
+}//namespace
 
 SkImageDecoder::Result SkGIFImageDecoder::onDecode(SkStream* sk_stream, SkBitmap* bm, Mode mode) {
 #if GIFLIB_MAJOR < 5