selftests/sysctl: Make sysctl test driver as a module

Fix config file to require CONFIG_TEST_SYSCTL=m instead of y
because this driver introduces a test sysctl interfaces which
are normally not used, and only used for the selftest.

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