dmx: Silence an unused-result warning

Modern glibc is very insistent that you care about whether write()
succeeds:

../hw/dmx/input/usb-keyboard.c: In function ‘kbdUSBCtrl’:
../hw/dmx/input/usb-keyboard.c:292:9: warning: ignoring return value of ‘write’, declared with attribute warn_unused_result [-Wunused-result]
         write(priv->fd, &event, sizeof(event));
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
(cherry picked from commit 17ad6e5d)