util: include unistd.h, which may be required for usleep prototype

util: include unistd.h, which may be required for usleep prototype

This seems to be dropped in 222a2fb9 "util: move os_time.[ch] to src/util"

../../../src/util/os_time.c: In function ‘os_time_sleep’:
../../../src/util/os_time.c:104:4: error: implicit declaration of function ‘usleep’ [-Werror=implicit-function-declaration]

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>