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>

drivers/staging/greybus/tools/loopback_test.c

@@ -168,7 +168,7 @@ GET_AVG(latency_avg);
 GET_AVG(apbridge_unipro_latency_avg);
 GET_AVG(gbphy_firmware_latency_avg);
 
-void abort()
+void abort(void)
 {
 	_exit(1);
 }