Update clang/test/PCH/single-token-macro.c corresponding to r206791.

__stdcall is defined as __attribute__((__stdcall__)) for targeting mingw32.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206870 91177308-0d34-0410-b5e6-96231b3b80d8