ncurses: serialize the tinfo build a little bit

Move ncurses_dll.h to GENHDRS to start with; it's been generated from
ncurses_dll.h.in for years, so it's not actually in a different category
than all of the other GENHDRS. Slap an .ORDER on it to ensure that we
build ncurses_dll.h and curses.h before any *.c gets compiled.

This should sufficiently address a build race seen downstream where
ncurses_dll.h is present but not yet populated.

Reviewed by: bapt
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D43540