usb: gadget: configfs: include appropriate header file in configfs.c

Include appropriate header file drivers/usb/gadget/configfs.h in
gadget/configfs.c because function unregister_gadget_item() has its
prototype declaration in gadget/configfs.h.

This eliminates the following warning in gadget/configfs.c:
drivers/usb/gadget/configfs.c:994:6: warning: no previous prototype for ‘unregister_gadget_item’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <[e-mail]>
Reviewed-by: Josh Triplett <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/gadget/configfs.c

···
 #include <linux/device.h>
 #include <linux/usb/composite.h>
 #include <linux/usb/gadget_configfs.h>
+#include "configfs.h"
 
 int check_user_usb_string(const char *name,
 		struct usb_gadget_strings *stringtab_dev)