Use POSIX-specified <poll.h> over <sys/poll.h>

POSIX specifies that <poll.h> is the correct header to include for
poll(), so let's do that instead.

https://bugzilla.gnome.org/show_bug.cgi?id=141251

tests/timeloop-basic.c

@@ -8,7 +8,7 @@
 #include <unistd.h>
 #include <sys/resource.h>
 #include <sys/time.h>
-#include <sys/poll.h>
+#include <poll.h>
 
 #define TRUE 1
 #define FALSE 0