use <poll.h> instead of <sys/poll.h>

The manpage of poll(2) states that the prototype of poll is defined
in <poll.h>. Use that header file instead of <sys/poll.h> to allow
compilation against musl-libc.

Signed-off-by: Romain Naour <romain.naour@smile.fr>

hal/ethernet/linux/ethernet_linux.c

@@ -23,7 +23,7 @@
 
 #include <sys/socket.h>
 #include <sys/ioctl.h>
-#include <sys/poll.h>
+#include <poll.h>
 #include <linux/if_packet.h>
 #include <linux/if_ether.h>
 #include <linux/if_arp.h>