x11/swaylock: unbreak on DragonFly

../loop.c: In function 'loop_poll':
../loop.c:83:2: error: implicit declaration of function 'poll'; did you mean 'powl'? [-Werror=implicit-function-declaration]

poll(loop->fds, loop->fd_length, ms);
^~~~
powl

Reported by: DPorts (muscles)