[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>