Remove double underscore in header guards (.h & __vtk*_h form)

Used the following command:

find . \( -iname *.h -and -type f \) -exec perl -pi -w -e 's/__vtk(.*)_h/vtk$1_h/g;' {} \;

Fixes many, but not all, clang -Wreserved-id-macro warnings.

Change-Id: I694ff053cb204c70e37b261296f496064d91fc07