This ensures that bitbake only attempt to inherit the qt5 classes if the
qt5-layer layer is present.  This also removes the inherit for qmake5_base
as cmake_qt5 inherits it as a dependency.  This prevents a parse error:

    Could not inherit file classes/qmake5_base.bbclass
Signed-off-by: Rob Woolley <rob.woolley@windriver.com>