C++ bindings: specify namespace on return type of get_ctx() method
C++ bindings: specify namespace on return type of get_ctx() method

Like the previous commit, this commits partially reverts
isl-0.18-1068-g85eb4b7f (drop namespace qualifier in generated
C++ bindings, Tue Jun 12 22:40:21 2018 +0200).
Again, the purpose is to avoid ambiguities
when the get_ctx() method is renamed to ctx().
For consistency with the previous commit, a namespace specifier
is added rather than adding the "class" keyword.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl