core: Fix ABI regression in recent commit.

* src/gpgme.h.in (_gpgme_op_import_result): Move new field
'skipped_v3_keys' to the end.

The ABI break has not made it into a release.

Also document the new field.

• Fixes-commit: a630a1e3e74c926163864b013cb164b4cd1866fc
• Signed-off-by: Werner Koch <wk@gnupg.org>