glsl2: Remove unnecessary use of 'struct' before type names

glsl2: Remove unnecessary use of 'struct' before type names

In C++ you don't have to say 'struct' or 'class' if the declaration of
the type has been seen.  Some compilers will complain if you use
'struct' when 'class' should have been used and vice versa.

Fixes bugzilla #29539.