common.c: fix compilation on OpenBSD

common.c: fix compilation on OpenBSD

cc1: warnings being treated as errors
common.c: In function 'set_sock_opts':
common.c:1572: warning: unused variable 'tcp_keepintvl'
common.c:1571: warning: unused variable 'tcp_keepidle'
*** Error 1 in target 'common.lo'
*** Error 1 in src/daemon (Makefile:1048 'common.lo': @echo "  CC      " common.lo;depbase=`echo common.lo | sed 's|[^/]*$|.deps/&|;s|\.lo$|...)
*** Error 1 in src (Makefile:5007 'all-recursive')
*** Error 2 in src (Makefile:3284 'all')
*** Error 1 in /root/src/collectd (Makefile:611 'all-recursive')