Fix test from r200979 on Windows

Hopefully the last tweak needed to get this test working everywhere.

Remove matching of the prefix of sys_header.h, which was never the point
of the test anyway.  This avoids dealing with path separators.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200987 91177308-0d34-0410-b5e6-96231b3b80d8