driver core: export subsys_virtual_register

Modules want to call this function, so it needs to be exported.
Reported-by: Daniel Mack <zonque@gmail.com>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/bus.c

@@ -1295,6 +1295,7 @@ int subsys_virtual_register(struct bus_type *subsys,
 
 	return subsys_register(subsys, groups, virtual_dir);
 }
+EXPORT_SYMBOL_GPL(subsys_virtual_register);
 
 int __init buses_init(void)
 {