Pre-updated invalid-target.rs test (#929)

This PR adds the commented out use spirv_std::spirv; to invalid-target.rs in preparation for #926

Author: Sylvester Hesp

tests/ui/spirv-attr/invalid-target.rs

@@ -30,6 +30,8 @@
 //   * 6 on `_entry_param`
 //   * 1 on `_closure`
 
+//use spirv_std::spirv;
+
 #[spirv(
     sampler, block, sampled_image, generic_image_type, // struct-only
     vertex, // fn-only