add png to makefile for linux



git-svn-id: http://skia.googlecode.com/svn/trunk@551 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/Makefile b/Makefile
index afc62c9..aebe467 100644
--- a/Makefile
+++ b/Makefile
@@ -73,6 +73,7 @@
     # these are our registry-based factories
 	SRC_LIST += src/images/SkImageDecoder_Factory.cpp
 	SRC_LIST += src/images/SkImageEncoder_Factory.cpp
+        SRC_LIST += src/images/SkImageDecoder_libpng.cpp
     # support files
 	SRC_LIST += src/images/SkScaledBitmapSampler.cpp
 endif