fixup??? compat/mingw: split out POSIX-related bits

A simple `#include "posix.h"` without dropping `-Icompat` from the
command-line will find `compat/posix.h`. In
`compat/mingw/compat-util.h`, that is not what we want to include, but
`compat/mingw/posix.h`.

In the long run we may want to reconsider using identical basenames
multiple times. This also helps with `gdb` where it is hard to
impossible to set breakpoints or list code in files with files with
filenames that are reused in other directories.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

compat/mingw/compat-util.h

@@ -1,7 +1,7 @@
 #ifndef COMPAT_MINGW_COMPAT_UTIL_H
 #define COMPAT_MINGW_COMPAT_UTIL_H
 
-#include "posix.h"
+#include "compat/mingw/posix.h"
 
 extern int core_fscache;
 int are_long_paths_enabled(void);