display-commonsys-intf: Define a soong namespace

Legacy platforms use their own set of headers and libraries
within hardware/qcom-caf/msm[0-9]+/display, so we need to hide
this with a soong namespace

Change-Id: I1f3a74c81cf3893d17c18a58f5a3b6e022e54eac
diff --git a/Android.bp b/Android.bp
index 408e904..22955cc 100644
--- a/Android.bp
+++ b/Android.bp
@@ -1,3 +1,6 @@
+soong_namespace {
+}
+
 cc_library_headers {
     name: "display_intf_headers",
     vendor_available: true,