fix missing include

struct timeval wasn't available, probably depending on libknot version,
breaking e.g. the Docker build.

lib/cache.h

@@ -17,6 +17,7 @@
 #pragma once
 
 #include <libknot/rrset.h>
+#include <sys/time.h>
 #include "lib/cdb.h"
 #include "lib/defines.h"
 #include "contrib/ucw/config.h" /*uint*/