resolver: fix the build of some ports, broken by r289315

r289315 required time_t and struct timespec to be defined before
including <resolv.h>.  This broke the build of net-mgmt/sx, at least.

Include <sys/timespec.h> in resolv.h to fix this with minimal pollution.

Reported by:	Raphael Kubo da Costa <rakuco>
MFC after:	3 days
Sponsored by:	Dell Inc.

include/resolv.h

@@ -57,6 +57,7 @@
 #include <sys/types.h>
 #include <sys/cdefs.h>
 #include <sys/socket.h>
+#include <sys/timespec.h>
 #include <stdio.h>
 #include <arpa/nameser.h>