Add -Wunknown-sanitizers to standalone builds, fix dbg build

Change-Id: Icbadd35c20a68b8fafbb56f88edcb3444e9d2f73
diff --git a/gn/standalone/BUILD.gn b/gn/standalone/BUILD.gn
index c7ef2ea..e88053b 100644
--- a/gn/standalone/BUILD.gn
+++ b/gn/standalone/BUILD.gn
@@ -35,6 +35,7 @@
       "-Wno-gnu-zero-variadic-macro-arguments",
       "-Wno-padded",
       "-Wno-reserved-id-macro",
+      "-Wno-unknown-sanitizers",
     ]
   }
 }