Include gif image decoding in images/ on linux.

tools/skimage_main.cpp

Index: tools/skimage_main.cpp
diff --git a/tools/skimage_main.cpp b/tools/skimage_main.cpp
index dc5bdd42607be18b3ad55cc87e0f14977d49800f..b486d5d1a3fd45cab4a0af407719e5eda88550ca 100644
--- a/tools/skimage_main.cpp
+++ b/tools/skimage_main.cpp
@@ -159,8 +159,15 @@ int tool_main(int argc, char** argv) {
 void forceLinking();
 
 void forceLinking() {
+    // This function leaks, but that is okay because it is not intended
+    // to be called. It is only here so that the linker will include the
+    // decoders.
     SkDEBUGCODE(SkImageDecoder *creator = ) CreateJPEGImageDecoder();
     SkASSERT(creator);
+#ifdef SK_BUILD_FOR_UNIX
+    SkDEBUGCODE(creator = ) CreateGIFImageDecoder();
+    SkASSERT(creator);
+#endif
 }
 
 #if !defined SK_BUILD_FOR_IOS