Add export macro to template definition.
Add export macro to template definition.

While this is somewhat nonsensical, it's required to properly
export instantiations of the template on older clang versions.
Without it, undefined symbol errors were produced from:

$ clang --version
Apple clang version 3.1 (tags/Apple/clang-318.0.58) (based on LLVM 3.1svn)
Target: x86_64-apple-darwin11.4.2
Thread model: posix

when linking to vtkCommonCore.

sumedhasingla/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,sumedhasingla/VTK,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,sumedhasingla/VTK,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,sumedhasingla/VTK,sumedhasingla/VTK,sumedhasingla/VTK,sumedhasingla/VTK,sumedhasingla/VTK