connector: fix defined but not used warning

Fix a build warning in connector.c when CONFIG_PROC_FS is not enabled
by marking the unused function as __maybe_unused.

../drivers/connector/connector.c:242:12: warning: 'cn_proc_show' defined but not used [-Wunused-function]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Evgeniy Polyakov <zbr@ioremap.net>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>