fix CMake error on Linux

Otherwise fails with:

-- Found Qt4: /usr/bin/qmake-qt4 (found version "4.8.4")
-- Found Qt4: /usr/bin/qmake-qt4 (found version "4.8.4")
-- Auto-add plugin subdir:/home/thomas/cloudcompare/qCC/plugins/qKinect
-- Auto-add plugin subdir:/home/thomas/cloudcompare/qCC/plugins/qDummyPlugin
-- Auto-add plugin subdir:/home/thomas/cloudcompare/qCC/plugins/qHPR
-- Auto-add plugin subdir:/home/thomas/cloudcompare/qCC/plugins/qRANSAC_SD
-- Auto-add plugin subdir:/home/thomas/cloudcompare/qCC/plugins/qPCV
-- Auto-add plugin subdir:/home/thomas/cloudcompare/qCC/plugins/qBlur
-- Auto-add plugin subdir:/home/thomas/cloudcompare/qCC/plugins/qSSAO
-- Auto-add plugin subdir:/home/thomas/cloudcompare/qCC/plugins/qEDL
-- Auto-add plugin subdir:/home/thomas/cloudcompare/qCC/plugins/qPoissonRecon
-- Auto-add plugin subdir:/home/thomas/cloudcompare/qCC/plugins/qPCL
-- Auto-add plugin subdir:/home/thomas/cloudcompare/qCC/plugins/CMakeFiles
CMake Error at qCC/plugins/CMakeLists.txt:12 (add_subdirectory):
  The source directory

    /home/thomas/cloudcompare/qCC/plugins/CMakeFiles

  does not contain a CMakeLists.txt file.

-- Configuring incomplete, errors occurred!

(cmake version 2.8.10.2)