Use kmod.opts.mk in sys/modules/tcp/Makefile to fix standalone builds.

This was included in the original commit to head that added use of
KERN_OPTS (r361638) but was left out of the MFC since kmod.opts.mk
hadn't been MFC'd at the time. It has since been merged.

This is a direct commit to stable/11 and stable/12.

Reported by: Eugene Grosbein <eugen@grosbein.net>