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