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_skia b/gyp_skia
index 75a3a21..cf55084 100755
--- a/gyp_skia
+++ b/gyp_skia
@@ -44,6 +44,12 @@
     if os.path.realpath(path) not in specified_includes:
       result.append(path)
 
+  # Always include common.gypi.
+  # We 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.
+  AddInclude(os.path.join(gyp_config_dir, 'common.gypi'))
+
   return result
 
 if __name__ == '__main__':