x11-wm/picom: unbreak on DragonFly

In file included from ../src/log.h:8,

from ../src/backend/gl/glx.h:18,
from ../src/common.h:42,
from ../src/c2.c:35:

../src/compiler.h:98:11: fatal error: threads.h: No such file or directory

1. include <threads.h> ^~~~~~~~~~~

Reported by: DPorts