Don't require users of src/port/gettimeofday.c to initialize it.

Don't require users of src/port/gettimeofday.c to initialize it.

Commit 8001fe67a3d66c95861ce1f7075ef03953670d13 introduced this
requirement, but per discussion, we want to avoid requirements of
this type to make things easier on the calling code.  An especially
important consideration is that this may be used in frontend code,
not just the backend.

Asif Naeem, reviewed by Michael Paquier