Compilation of main branch broken due to missing GuiRunner.hh

[scpeters]

Environment

• OS Version: both Ubuntu 18.04 and homebrew CI builds
• Source or binary build? source builds of the main branch as of 64632e0

Description

• Expected behavior: compile successfully
• Actual behavior: compilation fails

Steps to reproduce

1. Install ignition-fortress dependencies
2. Build ign-launch main branch from source
3. observer compilation error

Output

• https://build.osrfoundation.org/job/ignition_launch-ci-main-bionic-amd64/14/console
• https://build.osrfoundation.org/job/ignition_launch-ci-main-homebrew-amd64/15/console

plugins/gazebo_gui/GazeboGui.cc:23:10: fatal error: 'ignition/gazebo/gui/GuiRunner.hh' file not found
#include <ignition/gazebo/gui/GuiRunner.hh>
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
make[2]: *** [plugins/gazebo_gui/CMakeFiles/ignition-launch-gazebogui.dir/GazeboGui.cc.o] Error 1

the GuiRunner.hh header file was removed from the public API in gazebosim/gz-sim#875

[scpeters]

fixed by #119