devel/libcwd: Unbreak build with GCC >= 8

conftest.cpp:11:36: error: return-statement with a value, in function returning 'void'

http://package22.nyi.freebsd.org/data/112amd64-default-PR231590/2018-10-29_06h48m05s/logs/errors/libcwd-1.0.6_3.log

PR: 231590