media: exynos4-is: Fix compile warning

Declare static on function 'fimc_isp_video_device_unregister'.

When VIDEO_EXYNOS4_ISP_DMA_CAPTURE=n, compiler warns about
warning: no previous prototype for function [-Wmissing-prototypes]

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Kwanghoon Son <k.son@samsung.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>