nir: add missing GLSL_TYPE_DOUBLE case in type_size()

To silence compiler warning about unhandled switch case.
v2: move GLSL_TYPE_DOUBLE to the "not reached" section, per Ilia.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/glsl/nir/nir_lower_io.c

@@ -69,6 +69,7 @@ type_size(const struct glsl_type *type)
       return 0;
    case GLSL_TYPE_VOID:
    case GLSL_TYPE_ERROR:
+   case GLSL_TYPE_DOUBLE:
       unreachable("not reached");
    }