PNP: Hide pnp_bus_type from the non-PNP code

The pnp_bus_type is defined only when CONFIG_PNP=y, while being
not guarded by ifdeffery in the header. Moreover, it's not used
outside of the PNP code. Move it to the internal header to make
sure no-one will try to (ab)use it.
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/pnp/base.h

@@ -6,6 +6,7 @@
 
 extern struct mutex pnp_lock;
 extern const struct attribute_group *pnp_dev_groups[];
+extern const struct bus_type pnp_bus_type;
 
 int pnp_register_protocol(struct pnp_protocol *protocol);
 void pnp_unregister_protocol(struct pnp_protocol *protocol);

include/linux/pnp.h

@@ -435,8 +435,6 @@ struct pnp_protocol {
 #define protocol_for_each_dev(protocol, dev)	\
 	list_for_each_entry(dev, &(protocol)->devices, protocol_list)
 
-extern const struct bus_type pnp_bus_type;
-
 #if defined(CONFIG_PNP)
 
 /* device management */