The correct location is <poll.h>, not <sys/poll.h>

--- a/snapper/SystemCmd.h	2014-02-05 14:22:06.000000000 +0100
+++ b/snapper/SystemCmd.h	2015-08-30 10:56:03.685179465 +0200
@@ -23,7 +23,7 @@
 #ifndef SNAPPER_SYSTEM_CMD_H
 #define SNAPPER_SYSTEM_CMD_H
 
-#include <sys/poll.h>
+#include <poll.h>
 #include <stdio.h>
 
 #include <string>