Add missing block decoration.

The uniform buffer in the test/cube.wgsl needs a block attribute in
order to translate correct.

Change-Id: I2a245cffbe9f4c66b9d6ebcd66617a7fbdd98af2
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/26922
Commit-Queue: David Neto <dneto@google.com>
Reviewed-by: David Neto <dneto@google.com>

test/cube.wgsl

@@ -16,7 +16,7 @@ entry_point vertex = vtx_main;
 entry_point fragment = frag_main;
 
 # Vertex shader
-type Uniforms = struct {
+type Uniforms = [[block]] struct {
   [[offset 0]] modelViewProjectionMatrix : mat4x4<f32>;
 };