flatpak: Install CMake config files

CMake config files are required for building OBS plugins with the
Flatpak as a runtime.

Add a post-install command that installs the `obs_libraries`
component, which provides the config files.

CI/flatpak/com.obsproject.Studio.json

@@ -444,6 +444,7 @@
         "-DYOUTUBE_SECRET_HASH=$YOUTUBE_SECRET_HASH"
       ],
       "post-install": [
+        "cmake --install . --component obs_libraries",
         "install -d /app/plugins",
         "install -d /app/extensions/Plugins"
       ],