Add numpy dependency to package.xml.

The build is currently failing due to the numpy import in the template
http://build.ros2.org/job/Dbin_uB64__rosidl_generator_py__ubuntu_bionic_amd64__binary/1/console

I'm not sure of the proper scope for this dependency so for this PR I've
opted to make it a blanket dependency in order to proceed with the
release absent feedback.

rosidl_generator_py/package.xml

@@ -20,6 +20,8 @@
   <buildtool_export_depend>rosidl_typesupport_c</buildtool_export_depend>
   <buildtool_export_depend>rosidl_typesupport_interface</buildtool_export_depend>
 
+  <depend>python3-numpy</depend>
+
   <exec_depend>rmw_implementation</exec_depend>
   <exec_depend>rmw_implementation_cmake</exec_depend>
   <exec_depend>rosidl_generator_c</exec_depend>