[media] pxa_camera: make soc_mbus_xlate_by_fourcc() static

As warned by smatch:

drivers/media/platform/pxa_camera.c:283:39: warning: no previous prototype for 'soc_mbus_xlate_by_fourcc' [-Wmissing-prototypes]
 const struct soc_camera_format_xlate *soc_mbus_xlate_by_fourcc(
                                       ^~~~~~~~~~~~~~~~~~~~~~~~
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/platform/pxa_camera.c

@@ -280,8 +280,9 @@ static const char *pxa_cam_driver_description = "PXA_Camera";
 /*
  * Format translation functions
  */
-const struct soc_camera_format_xlate *soc_mbus_xlate_by_fourcc(
-	struct soc_camera_format_xlate *user_formats, unsigned int fourcc)
+static const struct soc_camera_format_xlate
+*soc_mbus_xlate_by_fourcc(struct soc_camera_format_xlate *user_formats,
+			  unsigned int fourcc)
 {
 	unsigned int i;