Update to github.com/mtrmac/gpgme v0.1.2

[mtrmac]

- What I did

This fixes CVE-2020-8945 by incorporating proglottis/gpgme#23 .

Other changes included by the rebase:

• Support for gpgme_off_t (~no-op with the RHEL 7 GPGME 1.3.2)
• Wrapping a few more GPGME functions (irrelevant if we don't call them)
• Better error reporting in Context.GetKey

Given how invasive the CVE fix is (affecting basically all binding code), it seems safer to just update the package (and be verifiably equivalent with upstream) than to backport and try to back out the few other changes.

- How I did it

Performed by updating vendor.conf, and

$ mkdir -p _build/src/github.com/docker
$ ln -s $(pwd) _build/src/github.com/docker/docker
$ GOPATH=$(pwd)/_build:$GOPATH vndr github.com/mtrmac/gpgme

- How to verify it

I have only checked that it compiles, via

$ mkdir -p _build/src/github.com/docker
$ ln -s $(pwd) _build/src/github.com/docker/docker
$ GOPATH=$(pwd)/_build:$GOPATH IAMSTATIC=false DOCKER_DEBUG=1 hack/make.sh dynbinary

- Description for the changelog Incorporated a fix for CVE-2020-8945 .

[mtrmac]

(Warning: I didn’t test one bit whether the result works.)