vgaarb.h: fix build warnings

Fix build warnings by providing a struct stub since no fields of
the struct are used:

include/linux/vgaarb.h:66:9: warning: 'struct pci_dev' declared inside parameter list
include/linux/vgaarb.h:66:9: warning: its scope is only this definition or declaration, which is probably not what you want
include/linux/vgaarb.h:99:34: warning: 'struct pci_dev' declared inside parameter list
include/linux/vgaarb.h:109:6: warning: 'struct pci_dev' declared inside parameter list
include/linux/vgaarb.h:121:8: warning: 'struct pci_dev' declared inside parameter list
include/linux/vgaarb.h:140:37: warning: 'struct pci_dev' declared inside parameter list

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>

include/linux/vgaarb.h

@@ -47,6 +47,8 @@
  */
 #define VGA_DEFAULT_DEVICE     (NULL)
 
+struct pci_dev;
+
 /* For use by clients */
 
 /**