.lgtm.yml: Prepare for Ubuntu 19.10
.lgtm.yml: Prepare for Ubuntu 19.10

We'll soon upgrade the lgtm.com build environment to Ubuntu 19.10, where 
`libg15daemon-client-dev` is no longer available, which makes the build
fail because it can't find `g15daemon_client.h`. Adding `no-g15` makes 
the build succeed.
Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble