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.comSigned-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>

lib/test_string.c

@@ -179,6 +179,10 @@ static __init int strnchr_selftest(void)
 	return 0;
 }
 
+static __exit void string_selftest_remove(void)
+{
+}
+
 static __init int string_selftest_init(void)
 {
 	int test, subtest;
@@ -216,4 +220,5 @@ static __init int string_selftest_init(void)
 }
 
 module_init(string_selftest_init);
+module_exit(string_selftest_remove);
 MODULE_LICENSE("GPL v2");