games/starfighter: fix build on GCC architectures

Set USE_CSTD=c99 to fix build issue when using base GCC:
gfx.c:922: error: 'for' loop initial declaration used outside C99 mode

PR: 240691
Approved by: portmgr (blanket: build fix), linimon (mentor)
Differential Revision: https://reviews.freebsd.org/D21722