Bug#917461: proofgeneral does not work in emacs-26

[Takeshi Soejima]

Dear Maintainer,

This bug was fixed by 'commit b3ff384' on git.

  https://github.com/ProofGeneral/PG/issues/355

  
https://github.com/ProofGeneral/PG/commit/b3ff384a564a0d5a4206c1eb1e2eaa06da6f909a

I wish this package to be included in Buster again.