[tint][gn] Fix unused variable warnings

When 'tint_has_fuzzers' is false.

Change-Id: I76bdf73cc51cce85b63f8c8b5dfab908bd8dd4c3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/157861
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Kokoro: Ben Clayton <bclayton@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>

src/tint/tint.gni

diff --git a/src/tint/tint.gni b/src/tint/tint.gni
index f9c3498..137def3 100644
--- a/src/tint/tint.gni
+++ b/src/tint/tint.gni
@@ -154,7 +154,11 @@
   }
 } else {
   template("tint_fuzz_source_set") {
+    not_needed(invoker, "*")
+    not_needed("*")
   }
   template("tint_fuzzer_test") {
+    not_needed(invoker, "*")
+    not_needed("*")
   }
 }