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

drivers/usb/gadget/f_ncm.c:1389:21: warning: symbol 'ncm_alloc' was not declared. Should it be static?
Signed-off-by: NJingoo Han <jg1.han@samsung.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>