usb: gadget: f_hid: hidg_alloc() can be static

drivers/usb/gadget/function/f_hid.c:852:21: sparse: symbol 'hidg_alloc' was not declared. Should it be static?

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>