Use VAR_ARRAYS on FreeBSD (same as audio/opus)

--- src/webrtc/src/third_party/opus/BUILD.gn	2021-03-15 17:51:55 UTC
+++ src/webrtc/src/third_party/opus/BUILD.gn
@@ -37,10 +37,11 @@ config("opus_private_config") {
     "OPUS_EXPORT=",
     "ENABLE_HARDENING",
 
+    "VAR_ARRAYS",
     # Prefer alloca() over variable length arrays which are often inefficient;
     # the opus code will automatically handle this correctly per-platform.
-    "USE_ALLOCA",
-    "HAVE_ALLOCA_H",
+#    "USE_ALLOCA",
+#    "HAVE_ALLOCA_H",
   ]
 
   include_dirs = [