util/rand_xor: add missing include statements

util/rand_xor: add missing include statements

Fixes for:

src/util/rand_xor.c:60:13: error: implicit declaration of function 'open' [-Werror=implicit-function-declaration]
    int fd = open("/dev/urandom", O_RDONLY);
             ^~~~
src/util/rand_xor.c:60:34: error: 'O_RDONLY' undeclared (first use in this function)
    int fd = open("/dev/urandom", O_RDONLY);
                                  ^~~~~~~~

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>