compositor: fix compiler warning due to c99-ism
Reported on IRC (don't remember context, might have been mingw related).
../gst/compositor/compositor.c: In function '_should_draw_background':
../gst/compositor/compositor.c:854:3: error: 'for' loop initial declarations are only allowed in C99 mode
../gst/compositor/compositor.c:854:3: note: use option -std=c99 or -std=gnu99 to compile your code