Simplify the enumerator_list rule for enum definitions.
Simplify the enumerator_list rule for enum definitions.

Even though C++03 disallows trailing commas in enum definitions,
all compilers allow them, so we must do the same.  The simplest
way to do this is to allow enumerator_definition to be empty,
rather than to tack on another rule.

Change-Id: I61a7fbcf1d9696b9d357f87bd6805c3873466398

johnkit/vtk-dev,demarle/VTK,sankhesh/VTK,sumedhasingla/VTK,hendradarwin/VTK,jmerkow/VTK,msmolens/VTK,gram526/VTK,jmerkow/VTK,berendkleinhaneveld/VTK,keithroe/vtkoptix,SimVascular/VTK,mspark93/VTK,johnkit/vtk-dev,SimVascular/VTK,ashray/VTK-EVM,johnkit/vtk-dev,gram526/VTK,candy7393/VTK,berendkleinhaneveld/VTK,sumedhasingla/VTK,mspark93/VTK,berendkleinhaneveld/VTK,berendkleinhaneveld/VTK,demarle/VTK,sankhesh/VTK,jmerkow/VTK,candy7393/VTK,johnkit/vtk-dev,berendkleinhaneveld/VTK,hendradarwin/VTK,hendradarwin/VTK,SimVascular/VTK,gram526/VTK,johnkit/vtk-dev,gram526/VTK,keithroe/vtkoptix,msmolens/VTK,keithroe/vtkoptix,candy7393/VTK,sankhesh/VTK,msmolens/VTK,jmerkow/VTK,sankhesh/VTK,johnkit/vtk-dev,keithroe/vtkoptix,berendkleinhaneveld/VTK,ashray/VTK-EVM,mspark93/VTK,jmerkow/VTK,jmerkow/VTK,ashray/VTK-EVM,jmerkow/VTK,gram526/VTK,candy7393/VTK,gram526/VTK,hendradarwin/VTK,SimVascular/VTK,ashray/VTK-EVM,ashray/VTK-EVM,johnkit/vtk-dev,gram526/VTK,ashray/VTK-EVM,sumedhasingla/VTK,sumedhasingla/VTK,mspark93/VTK,sankhesh/VTK,hendradarwin/VTK,sumedhasingla/VTK,jmerkow/VTK,demarle/VTK,ashray/VTK-EVM,sankhesh/VTK,msmolens/VTK,candy7393/VTK,candy7393/VTK,keithroe/vtkoptix,hendradarwin/VTK,msmolens/VTK,keithroe/vtkoptix,mspark93/VTK,SimVascular/VTK,candy7393/VTK,demarle/VTK,demarle/VTK,demarle/VTK,keithroe/vtkoptix,mspark93/VTK,mspark93/VTK,msmolens/VTK,sankhesh/VTK,SimVascular/VTK,sumedhasingla/VTK,keithroe/vtkoptix,msmolens/VTK,berendkleinhaneveld/VTK,gram526/VTK,msmolens/VTK,SimVascular/VTK,sumedhasingla/VTK,SimVascular/VTK,hendradarwin/VTK,candy7393/VTK,demarle/VTK,sumedhasingla/VTK,demarle/VTK,ashray/VTK-EVM,mspark93/VTK,sankhesh/VTK