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.