v4l2-common: warning fix (W=1): add a missed function prototype

changeset d1e9b7c1 added a new ancilliary function to API, but forgot
to add it to the corresponding header file.

drivers/media/v4l2-core/v4l2-ctrls.c:568:18: warning: no previous prototype for 'v4l2_ctrl_get_int_menu' [-Wmissing-prototypes]
 const s64 const *v4l2_ctrl_get_int_menu(u32 id, u32 *len)

Cc: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>