Remove stale common/game include path

[Marzac]

Summary

• CMakeLists.txt and waller.pro both referenced ../common/game, a directory that doesn't exist in the repo (leftover from before the qmake -> CMake migration).
• Removed the stale include path from both build files. No other references to common/game or common/network exist in the repo.

Test plan

• Configure and build with CMake to confirm the editor still compiles without the include path.