cmake: add missing argument to add_subdirectory

We need the EXCLUDE_FROM_ALL argument when adding git submodules via
add_subdirectory in the CMakeLists file. Without it, the "install" make
target for Jami will also install the submodule's files.

GitLab: #1947
Change-Id: I477c4733951ac5a9fa2b6a52a1463c4a585ab8b7