libudev: make valgrind happy

libudev: make valgrind happy

Warning: noted but unhandled ioctl 0x641e with no size/direction hints
  Syscall param socketcall.setsockopt(optval) points to uninitialised byte(s)
  at 0x6A30BAA: setsockopt (syscall-template.S:82)
  by 0x4E36E1C: udev_monitor_filter_update (libudev-monitor.c:347)
  by 0x4E36ED8: udev_monitor_enable_receiving (libudev-monitor.c:373)
  by 0x8A9E68E: I830ScreenInit (intel_driver.c:838)
  by 0x42EE9E: AddScreen (dispatch.c:3890)
  by 0x471561: InitOutput (xf86Init.c:738)
  by 0x422C18: main (main.c:205)
  Location 0x7ff0000c2 is 2 bytes inside local var "filter"
  declared at libudev-monitor.c:256, in frame #1 of thread 1
  Uninitialised value was created by a stack allocation
  at 0x4E30860: ??? (in /lib64/libudev.so.0.10.0)

Thanks to Zdenek Kabelac.