Add SPIRV-Tools to GN header check.

Bug: None
Change-Id: Ibd16148a8b641131b8d5c22e9a58f77453f09f69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1784622Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693865}

.gn

@@ -323,8 +323,7 @@ check_targets = [
   #"//third_party/*",  # May not ever want this.
   "//third_party:freetype_harfbuzz",
   "//third_party/Python-Markdown/*",
-
-  # "//third_party/SPIRV-Tools/*",  # 30ish errors
+  "//third_party/SPIRV-Tools/*",
   "//third_party/abseil-cpp/*",
   "//third_party/accessibility-audit/*",
   "//third_party/accessibility_test_framework/*",