root_generate_dictionary: do not search for headers with find_file()

This leads to errors due to absolute paths if ROOT is already
installed in the configured prefix (e.g. /usr/local):

CMake Error at cmake/modules/RootNewMacros.cmake:288 (message):
  Header path
  '/usr/local/include/ROOT/RVirtualCanvasPainter.hxx'
   /usr/local/include/ROOT/RVirtualCanvasPainter.hxx
  is not relative!
Call Stack (most recent call first):
  cmake/modules/RootNewMacros.cmake:968 (ROOT_GENERATE_DICTIONARY)
  graf2d/gpadv7/CMakeLists.txt:6 (ROOT_STANDARD_LIBRARY_PACKAGE)

However, we only want this behavior for ROOT itself and not dependent
projects, which still use globbing in their build systems.