Add a guest only build rule

BUG: 78296257
Test: Local build after the wifi_relay split
Change-Id: I5f0488e53f309541f770d50845feeb66e20d3ba6
diff --git a/Android.bp b/Android.bp
index a1c05ac..1b3a986 100644
--- a/Android.bp
+++ b/Android.bp
@@ -70,6 +70,11 @@
 }
 
 cc_defaults {
+    name: "cuttlefish_guest_only",
+    defaults: ["cuttlefish_base"],
+}
+
+cc_defaults {
     name: "cuttlefish_host_only",
     device_supported: false,
     host_supported: true,