lib: Make test_sysctl initialized as module

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.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>