gnupg2: Fix generation of 8192 keys

In #4201 support for generating 8192-bit keys with GNUPG was added but
since 2.0.25 the code has changed and "normalises" the key, see this
commit <http://git.gnupg.org/cgi-bin/gitweb.cgi?p=gnupg.git;a=commit;h=48d92bcc8870f5750fb66351f3623f9d874d08fa>.