Fix incorrect include

In file included from ../clients/multi-resource.c:38:
/usr/include/sys/poll.h:1:2: warning: #warning redirecting incorrect #include <sys/poll.h> to <poll.h> [-Wcpp]
 #warning redirecting incorrect #include <sys/poll.h> to <poll.h>
  ^~~~~~~

(cherry picked from commit 923a1e96)