[media] mem2mem_testdev: Make m2mtest_dev_release function static

Fixes the following warning:
drivers/media/platform/mem2mem_testdev.c:73:6: warning:
symbol 'm2mtest_dev_release' was not declared. Should it be static?
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/platform/mem2mem_testdev.c

@@ -70,7 +70,7 @@ MODULE_VERSION("0.1.1");
 	v4l2_dbg(1, 1, &dev->v4l2_dev, "%s: " fmt, __func__, ## arg)
 
 
-void m2mtest_dev_release(struct device *dev)
+static void m2mtest_dev_release(struct device *dev)
 {}
 
 static struct platform_device m2mtest_pdev = {