audio/festival: unbreak on most non-x86 archs

For example, after r450862 mips64 still failed:

gmake[1]: Entering directory '/wrkdirs/usr/ports/audio/festival/work/speech_tools'
Check system type
../config/config:197: ../config/systems/mips_unknown.mak: No such file or directory
gmake[1]: *** No rule to make target '../config/systems/mips_unknown.mak'. Stop.