weston: added missing header time.h

without it I can't built weston on alpinelinux

Reviewed-by: Daniel Stone <daniels@collabora.com>

tests/timespec-test.c

@@ -32,6 +32,7 @@
 #include <assert.h>
 #include <errno.h>
 #include <unistd.h>
+#include <time.h>
 
 #include "timespec-util.h"