Fix failure-to-compile-standalone in ecpg's dt.h.

Fix failure-to-compile-standalone in ecpg's dt.h.

This has to have <time.h>, or the references to "struct tm" don't
mean what they should.

We have some other recently-introduced issues of the same ilk,
but this one seems old.  No backpatch though, as it's only a
latent problem for most purposes.