net/libusrsctp: fix build on GCC architectures

Disable -Werror:
/wrkdirs/usr/ports/net/libusrsctp/work/usrsctp-0.9.3.0-856-g66a25d7/usrsctplib/netinet/sctp_auth.c:197: warning: declaration of 'index' shadows a global declaration

MFH: 2020Q3 (fix build blanket)