audio/protracker: fix build on GCC architectures

Add USE_CSTD=c99 to fix build:
/wrkdirs/usr/ports/audio/protracker/work/protracker-2.3d.r191/src/pt_visuals.c:1272: error: 'for' loop initial declaration used outside C99 mode

PR: 240703
Approved by: portmgr (blanket: build fix), linimon (mentor)