test/Modules/compiler_builtins.m: Appease Cygwin to add -D__need_wint_t.

On Cygwin, at first, <stddef.h> is included without __need_wint_t.
Next, <stddef.h> is included with __need_wint_t, though Modules feature would not process <stddef.h> twice.
Then, wint_t is not found in system headers.

llvm-svn: 149500