doc: Do not install gnupg.7.html into usr/share/man/manh/

* doc/Makefile.am: Ship gnupg.7.html with other html, not with
manpages.

Without this change, gnupg.7.html gets placed in /usr/share/manh/
Since it can't be correctly rendered by groff, this is undesirable.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>