lib/test_string.c: allow module removal

The test_string module can't be removed because it lacks an exit hook.
Since there is no reason for it to be permanent, add an empty one to allow
module removal.

Link: https://lkml.kernel.org/r/20210616234503.28678-1-mcroce@linux.microsoft.com


Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>