Minor fix for header parsing issues

git-svn-id: https://plugins.svn.wordpress.org/theme-check/trunk@1013095 b8457f37-d9ea-0310-8a92-e5e31aec5664
Use preg_match to check for function calls more accurately.

git-svn-id: https://plugins.svn.wordpress.org/theme-check/trunk@1013097 b8457f37-d9ea-0310-8a92-e5e31aec5664