Add missing @interpolate(flat) to validation test.

(Note that I've added the decoration to all types, integer or not.
This should make no difference for the purpose of this test.)

Bug: dawn:1271
Change-Id: I77a4c3ac25ad360ea59352499d97148c068ceda1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/78204
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Stephen White <senorblanco@chromium.org>

src/tests/unittests/validation/RenderPipelineValidationTests.cpp

@@ -1334,7 +1334,8 @@ TEST_F(InterStageVariableMatchingValidationTest, DifferentTypeAtSameLocation) {
         std::string interfaceDeclaration;
         {
             std::ostringstream sstream;
-            sstream << "struct A { @location(0) a: " << kTypes[i] << ";" << std::endl;
+            sstream << "struct A { @location(0) @interpolate(flat) a: " << kTypes[i] << ";"
+                    << std::endl;
             interfaceDeclaration = sstream.str();
         }
         {