[clang] Try to unbreak test/C/drs/dr324.c on Windows

The test uses

    #pragma GCC dependency "oops\..\dr0xx.c"

to test preprocessor "tokenization obscurities". However, that
pragma also emits a warning if the refered file is newer than the
current file -- and d694e249 edited dr0xx.c, causing this warning
to fire.

As workaround, touch this file, so that it's newer than dr0xx.c again.