Don't try to compile .mk files when building shared libraries

(to exclude third_party/skia/src/core/core_files.mk).
Review URL: http://codereview.chromium.org/115320

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@15999 0039d316-1c4b-4281-b951-d872f2087c98

site_scons/site_tools/chromium_builders.py

@@ -167,6 +167,7 @@ non_compilable_suffixes = {
         '.html',
         '.hxx',
         '.idl',
+        '.mk',
         '.js',
         '.rc',
     ]),