audio/fasttracker2: fix build with base GCC

Add USE_CSTD=c99 to fix build with base GCC:
/wrkdirs/usr/ports/audio/fasttracker2/work/ft2-clone-code/src/ft2_gui.c:431: error: 'for' loop initial declaration used outside C99 mode

PR: 240659
Approved by: ehaupt (maintainer), linimon (mentor)
Differential Revision: https://reviews.freebsd.org/D21702