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/[e-mail]
Signed-off-by: Matteo Croce <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

lib/test_string.c

···
 	return 0;
 }
 
+static __exit void string_selftest_remove(void)
+{
+}
+
 static __init int string_selftest_init(void)
 {
 	int test, subtest;
···
 }
 
 module_init(string_selftest_init);
+module_exit(string_selftest_remove);
 MODULE_LICENSE("GPL v2");