fix spirv-reader test: add Block decoration

Fixes a SPIR-V validation error.

Change-Id: I51bf887d1e2b55760e5512648074ae349a90a15e
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/74802
Auto-Submit: David Neto <dneto@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: David Neto <dneto@google.com>

src/reader/spirv/parser_impl_module_var_test.cc

diff --git a/src/reader/spirv/parser_impl_module_var_test.cc b/src/reader/spirv/parser_impl_module_var_test.cc
index df03435..a8c683c 100644
--- a/src/reader/spirv/parser_impl_module_var_test.cc
+++ b/src/reader/spirv/parser_impl_module_var_test.cc
@@ -370,6 +370,7 @@
   OpEntryPoint Vertex %main "main" %1
 
  ;  scramble the member indices
+  OpDecorate %10 Block
   OpMemberDecorate %10 0 BuiltIn ClipDistance
   OpMemberDecorate %10 1 BuiltIn CullDistance
   OpMemberDecorate %10 2 BuiltIn Position