net/linphone: Fix build when git is installed

* The upstream build includes a subdirectory that attempts to call git,
  expecting the source to be a git clone, so the build failed on a
  system that has git installed. Just exclude this subdirectory, as it's
  not used to build linphone itself (and therefore doesn't affect the
  outcome), patch suggested by hselasky in D35805.
* Take maintainership.

PR:		264734