glsl: Silence ignored qualifier warning

glsl: Silence ignored qualifier warning

I think the intention was to mark the "this" parameter as const, but
const goes on the other end to do that.

In file included from glsl_symbol_table.cpp:26:0:
ast.h:339:35: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
    const bool is_single_dimension()
                                   ^

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>