gbuild: UnoApi: fix rebuilds after IDL changes

There used to be a dependency between .urd file and .hpp/.hdl files but
that was removed; introduce a dependency between .idl files and
.hpp/.hdl files to replace it so headers and stuff depending on them get
rebuilt.

Change-Id: Ibf1fe1db08c36ce42c392d27b794c2bc2a33738c
(cherry picked from commit 0190a70e79efa437e1516e635f4f670d7a35bb2d)
Reviewed-on: https://gerrit.libreoffice.org/7547
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>