glsl: silence warning about unhandled ast_unsized_array_dim case in switch

Reviewed-by: Timothy Arceri <t_arceri@yahoo.com.au>

src/glsl/ast_to_hir.cpp

@@ -2017,6 +2017,9 @@ ast_expression::has_sequence_subexpression() const
 
    case ast_function_call:
       unreachable("should be handled by ast_function_expression::hir");
+
+   case ast_unsized_array_dim:
+      unreachable("ast_unsized_array_dim: Should never get here.");
    }
 
    return false;