CMake: multibuild script has a Release and Debug mixup due missing 'NOT' statement in line 17

the multibuild cmake script has a Release Debug mixup due missing 'NOT' statement in line 17

see: https://github.com/KDAB/GammaRay/blob/73a102f50f0c4f9482f90b84814aa9cd812c3136/multibuild/CMakeLists.txt#L17-L20