tests: Fixes building test-util-filemonitor.c on msys2/mingw

tests: Fixes building test-util-filemonitor.c on msys2/mingw

Fixes the following compiling error:
../tests/test-util-filemonitor.c: In function 'test_file_monitor_events':
../tests/test-util-filemonitor.c:620:17: error: too many arguments to function 'mkdir'
  620 |             if (mkdir(pathsrc, 0700) < 0) {
      |                 ^~~~~
In file included from C:/CI-Tools/msys64/mingw64/x86_64-w64-mingw32/include/unistd.h:10,
                 from C:/work/xemu/qemu/include/qemu/osdep.h:93,
                 from ../tests/test-util-filemonitor.c:21:
C:/CI-Tools/msys64/mingw64/x86_64-w64-mingw32/include/io.h:282:15: note: declared here
  282 |   int __cdecl mkdir (const char *) __MINGW_ATTRIB_DEPRECATED_MSVC2005;
      |               ^~~~~

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Message-Id: <20200905203425.1470-1-luoyonggang@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>