usb: gadget: f_obex: Fix sparse warning

Make local symbol static in order to fix the following sparse
warning.

drivers/usb/gadget/f_obex.c:502:21: warning: symbol 'obex_alloc' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
1 file changed