security/snort2pfcd: fix build on GCC architectures

cc1: error: unrecognized command line option "-Wimplicit-fallthrough"
cc1: error: unrecognized command line option "-Wcovered-switch-default"
cc1: error: unrecognized command line option "-Wno-noexcept-type"
cc1: error: unrecognized command line option "-Wstring-conversion"

Tested to build with Clang.