Bring back support for legacy bt_config.xml

If a device migrates directly from L or earlier to a version using this
new implementation, all bt_config will be lost, requiring to repair to
the devices.

Change-Id: Ifdca0d8cad8efaa9f503ca40ac0725547a06f15c
diff --git a/btif/BUILD.gn b/btif/BUILD.gn
index d7171b0..f0d3ef8 100644
--- a/btif/BUILD.gn
+++ b/btif/BUILD.gn
@@ -80,6 +80,7 @@
     "//stack/a2dp",
     "//stack/btm",
     "//stack/include",
+    "//third_party/tinyxml2",
     "//include",
     "//udrv/include",
     "//utils/include",