Add missing place_tracker workspace package to top-level pubspec (#2739)

The package already uses `resolution: workspace` but isn't listed in the
top-level `pubspec.yaml`. This fixes that.

Author: guidezpl

pubspec.yaml

@@ -38,6 +38,7 @@ workspace:
   - navigation_and_routing
   - pedometer
   - pedometer/example
+  - place_tracker
   - platform_channels
   - platform_design
   - platform_view_swift