doc: Update gpl.texi to match version from gnupg

Somehow the doc/gpl.texi from gpgme and gnupg drifted out of sync.
This patch to gpgme's file brings it in line with gnupg's master
branch, and avoids the following errors during make:

./gpl.texi:667: @section seen before @end enumerate
./gpl.texi:724: unmatched `@end enumerate'
./gpl.texi:1: warning: node next Copying' in menu Concept Index'

and in sectioning `Function and Data Index' differ