cmake: Don't reject unknown cpp files that start with .

Some editors create hidden file backups in the same
directory as the file, and it's annoying when cmake
errors on them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216941 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed