devel/libudev-devd: fix build with gcc10

Don't set -Werror:
../udev-device.c: In function 'udev_device_new_from_devnum':
../udev-device.c:93:2: error: 'strncpy' output may be truncated copying 31 bytes from a string of length 78 [-Werror=stringop-truncation]

93 |  strncpy(devbuf, devpath + 1, 31);