driver core: remove unnecessary function extern declare

device_private_init is called only in core.c, extern declare is
unnecessary and make it static.

Cc: Greg Kroah-Hartman <[e-mail]>
Cc: "Rafael J. Wysocki" <[e-mail]>
Signed-off-by: Shaokun Zhang <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/base/base.h

···
 #define to_device_private_bus(obj)	\
 	container_of(obj, struct device_private, knode_bus)
 
-extern int device_private_init(struct device *dev);
-
 /* initialisation functions */
 extern int devices_init(void);
 extern int buses_init(void);

drivers/base/core.c

···
 	}
 }
 
-int device_private_init(struct device *dev)
+static int device_private_init(struct device *dev)
 {
 	dev->p = kzalloc(sizeof(*dev->p), GFP_KERNEL);
 	if (!dev->p)