When compiling this port with GCC, the following warning is treated as error:

exprfilter_scanner.rl:114:1: warning: narrowing conversion of '-1' from 'int' to 'char' inside { } [-Wnarrowing]

Add -Wno-error=narrowing to CXXFLAGS on GCC-based architectures to fix
the build.

PR: 235349
Submitted by: Piotr Kubaj
Approved by: portmgr (tier-2 blanket)