QT_QT_INCLUDE_DIR is not defined using homebrew QT

In any case point_cloud_editor doesn't need it.

The error is:

```
-- Found Doxygen: /usr/local/bin/doxygen (found version "1.8.5")
-- Found PCAP: /usr/lib/libpcap.dylib
CMake Error at apps/point_cloud_editor/CMakeLists.txt:93 (INCLUDE_DIRECTORIES):
  include_directories given empty-string as include directory.
```