Fix fallback_image example (#8968)

# Objective

Fixes #8967 

## Solution

I think this example was just missed in #5703. I made the same sort of
changes to `fallback_image` that were made in other examples in that PR.

Author: rparrett

assets/shaders/fallback_image_test.wgsl

@@ -1,5 +1,6 @@
 #import bevy_pbr::mesh_view_bindings
 #import bevy_pbr::mesh_bindings
+#import bevy_pbr::mesh_vertex_output MeshVertexOutput
 
 @group(1) @binding(0)
 var test_texture_1d: texture_1d<f32>;
@@ -31,9 +32,5 @@ var test_texture_3d: texture_3d<f32>;
 @group(1) @binding(11)
 var test_texture_3d_sampler: sampler;
 
-struct FragmentInput {
-    #import bevy_pbr::mesh_vertex_output
-};
-
 @fragment
-fn fragment(in: FragmentInput) {}
+fn fragment(in: MeshVertexOutput) {}