net/ndproxy: add -Wno-error=strict-prototypes to CFLAGS when using GCC

This is needed to fix build:
ndproxy.c:51: function declaration isn't a prototype

PR: 237296
Approved by: fbsd.bugzilla@fenyo.net (maintainer), mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20013