Add -fmodules-cache-path to test/Headers/c11.c

This started failing for me the last time someone modified the AST file
format. It would be nice if we could just have lit take care of the
module cache used during testing for us, but this helps in the meantime.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203511 91177308-0d34-0410-b5e6-96231b3b80d8