xen/xenbus: make frontend bus GPL

Make sure frontend xenbus has a GPL module license, so it can access
all the xenbus symbols exported GPL only.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
[corresponds to 525cbc8adcb5 in git://git.kernel.org/pub/scm/linux/kernel/git/jeremy/xen.git]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xenbus/xenbus_probe_frontend.c

@@ -293,3 +293,5 @@ static int __init boot_wait_for_devices(void)
 
 late_initcall(boot_wait_for_devices);
 #endif
+
+MODULE_LICENSE("GPL");