mark isl_basic_set_substitute static
mark isl_basic_set_substitute static

This function is only used from within isl_set_substitute and
never appeared in any public header.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

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