headless: Fix inconsistent header naming

Because of a missing 's' the generated client API was always considered out of
date.

BUG=669077

Review-Url: https://codereview.chromium.org/2531363002
Cr-Commit-Position: refs/heads/master@{#434699}
diff --git a/headless/BUILD.gn b/headless/BUILD.gn
index 9c9d392..5a59eac 100644
--- a/headless/BUILD.gn
+++ b/headless/BUILD.gn
@@ -137,7 +137,7 @@
     "lib/browser/devtools_api/domain_h.template",
     "lib/browser/devtools_api/domain_type_conversions_h.template",
     "lib/browser/devtools_api/domain_types_cc.template",
-    "lib/browser/devtools_api/domain_types_forward_declaration_h.template",
+    "lib/browser/devtools_api/domain_types_forward_declarations_h.template",
     "lib/browser/devtools_api/domain_types_h.template",
   ]