Add pkg-config as a buildtool dependency

[chapulina]

@nuclearsandwich kindly helped me investigate why the ros_ign packages are failing to build for Foxy and this is a promising band-aid solution.

[nuclearsandwich]

I think you also need this in ros_ign_gazebo based on the failures here: http://build.ros2.org/view/Fbin_uF64/job/Fbin_uF64__ros_ign_gazebo__ubuntu_focal_amd64__binary/

[nuclearsandwich]

If ign-cmake was a ROS package, the common practice would be to add a buildtool_export_depend on pkg-config so that any package downstream of ign-cmake got the dependency needed to use its bundled cmake modules.

It's not a ROS package so while that possibility is entertained this PR moves forward with a local solution to include pkg-config as a buildtool dependency in the packages which use ign-cmake bundled modules that expect pkg-config to be available.

[chapulina]

I think you also need this in ros_ign_gazebo

You're right, I forgot that also links against ign-transport. 02be7a7

[nuclearsandwich]

This should at least get us building as a temporary workaround if not the desired approach.