test_sysctl.c is expected to be used as a module, but since
it does not use module_init(), it never be registered as
a module and not appeared under /sys/module/.
In the result, the selftests/sysctl/sysctl.sh always fails
to find the test module and is skipped.
This makes test_sysctl.c initialized as a module by module_init()
and allow sysctl.sh to find the test module is loaded.