Make these ".h" files adopt the same policy the ".c" files already
follow: don't use <subsystem/...h> syntax for private interfaces.
If we ever get reviewed/supported "public" interfaces they should
come exclusively from some include/... directory; that'll be the
time to switch to <...> syntax for any subsystem's own interfaces.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>