CMake: Use a setup function for CMakeBuildConfigurationFactory

The class itself is still used by IosCMakeBuildConfigurationFactory.

Change-Id: I9fd979fc8e24c9f032b35c1f3444c28d06f86d0a
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildconfiguration.h

@@ -131,4 +131,6 @@ private:
     friend class Internal::CMakeProjectImporter;
 };
 
+namespace Internal { void setupCMakeBuildConfiguration(); }
+
 } // namespace CMakeProjectManager