Add explicit git dependency from ament_cmake_vendor_package (#554)

The vcstool package can be used with multiple package managers, not just
git. Best to be explicit about which of those package managers we want
to use.

Signed-off-by: Scott K Logan <logans@cottsay.net>

ament_cmake_vendor_package/package.xml

@@ -16,6 +16,7 @@
 
   <buildtool_export_depend>ament_cmake_export_dependencies</buildtool_export_depend>
   <buildtool_export_depend>ament_cmake_core</buildtool_export_depend>
+  <buildtool_export_depend>git</buildtool_export_depend>
   <buildtool_export_depend>python3-vcstool</buildtool_export_depend>
 
   <test_depend>ament_cmake_test</test_depend>