udev: fix compile warning

  CC     udev/udevd.o
In file included from udev/udev.h:27,
                 from udev/udevd.c:47:
./libudev/libudev-private.h: In function ‘udev_selinux_setfscreateconat’:
./libudev/libudev-private.h:230: warning: declaration of ‘dirfd’ shadows a global declaration
/usr/include/dirent.h:224: warning: shadowed declaration is here

Signed-off-by: Yin Kangkai <kangkai.yin@intel.com>
Signed-off-by: Martin Pitt <martin.pitt@ubuntu.com>