vga_switcheroo.h: fix pci_dev warning

Fix warnings on some architectures/configs (not on x86):

include/linux/vga_switcheroo.h:28:30: warning: 'struct pci_dev' declared inside parameter list [enabled by default]
include/linux/vga_switcheroo.h:28:30: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Cc:	Takashi Iwai <tiwai@suse.de>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

include/linux/vga_switcheroo.h

@@ -9,6 +9,8 @@
 
 #include <linux/fb.h>
 
+struct pci_dev;
+
 enum vga_switcheroo_state {
 	VGA_SWITCHEROO_OFF,
 	VGA_SWITCHEROO_ON,