clang/test/Preprocessor/iwithprefix.c: Tweak default includes not to use /usr/include.

You may see such a message on non-posix system;
  ignoring nonexistent directory "/usr/local/include"
  ignoring nonexistent directory "/usr/include"

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173525 91177308-0d34-0410-b5e6-96231b3b80d8