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>

src/glsl/ast.h

@@ -336,7 +336,7 @@ public:
       array_dimensions.push_tail(&dim->link);
    }
 
-   const bool is_single_dimension()
+   bool is_single_dimension() const
    {
       return this->array_dimensions.tail_pred->prev != NULL &&
              this->array_dimensions.tail_pred->prev->is_head_sentinel();