headless: Add a dummy include for version.h

headless/lib/browser/headless_browser_impl.h

Index: headless/lib/browser/headless_browser_impl.h
diff --git a/headless/lib/browser/headless_browser_impl.h b/headless/lib/browser/headless_browser_impl.h
index e622227f65b5f0e13e89224f3cf8d7fa4dc6338b..b108fad80cc2d0535a113059a57846df2e61192f 100644
--- a/headless/lib/browser/headless_browser_impl.h
+++ b/headless/lib/browser/headless_browser_impl.h
@@ -21,6 +21,9 @@
 #include "headless/lib/browser/headless_window_tree_host.h"
 #endif
 
+// Note: unused but needed to make sure this header gets generated.
+#include "headless/public/version.h"
+
 namespace headless {
 
 class HeadlessBrowserContextImpl;