staging: greybus: loopback_test: fix checkpatch bad function definition error

Fix the following Errors:
Bad function definition - void abort() should probably be void abort(void)

Signed-off-by: Abdul Rauf <abdulraufmujahid@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>