fix incorrect poll.h inclusion

sys/poll.h is a glibc legacy alias for poll.h.
the latter is specified by POSIX, the former not.
on glibc one of them just includes the other so it doesnt hurt.

src/core.c

@@ -27,7 +27,7 @@
 #include <arpa/inet.h>
 #include <sys/types.h>
 #include <sys/socket.h>
-#include <sys/poll.h>
+#include <poll.h>
 #include <sys/wait.h>
 #include <fcntl.h>
 #include <time.h>