build: Create an SWDB file during "make distcheck"
* Makefile.am (distcheck-hook): New.
The SWDB file is used to update the swdb.mac file from the
gnupg-doc repo which is the source for
https://gnupg.org/swdb.lst
https://gnupg.org/swdb.lst.sig
- Signed-off-by: Werner Koch <wk@gnupg.org>