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 <[e-mail]>
Signed-off-by: Kwanghoon Son <[e-mail]>
Signed-off-by: Sakari Ailus <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/platform/samsung/exynos4-is/fimc-isp-video.h

···
 	return 0;
 }
 
-void fimc_isp_video_device_unregister(struct fimc_isp *isp,
+static inline void fimc_isp_video_device_unregister(struct fimc_isp *isp,
 				enum v4l2_buf_type type)
 {
 }