Fixes the following sparse warning:

drivers/pci/hotplug/acpiphp_glue.c:923:6: warning:
 symbol 'acpiphp_drop_bridge' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>