Make all gyp targets automatically include common.gypi

Do this, rather than including common.gypi explicitly in all our gyp files, so that gyp files we use but do not maintain (e.g., third_party/externals/libjpeg/libjpeg.gyp) will include common.gypi too.
Review URL: https://codereview.appspot.com/5820068

git-svn-id: http://skia.googlecode.com/svn/trunk@3411 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gyp/android_system.gyp b/gyp/android_system.gyp
index 27a82f3..bbc1e69 100644
--- a/gyp/android_system.gyp
+++ b/gyp/android_system.gyp
@@ -15,7 +15,6 @@
 # variable expansion step for gyp happens after the includes are processed. 
 {
   'includes': [
-    'common.gypi',
     '../../android/gyp/android.gypi',
   ],
-}
\ No newline at end of file
+}