Add default_visibility for soong modules

So that we don't get surprise usages.

Bug: 348717861
Test: m nothing
Change-Id: I47319a727092c1bd936cca88f5713ee977a48b48
diff --git a/linkerconfig/proto/Android.bp b/linkerconfig/proto/Android.bp
index 754e7bf..a930502 100644
--- a/linkerconfig/proto/Android.bp
+++ b/linkerconfig/proto/Android.bp
@@ -15,6 +15,7 @@
         "//apex_available:platform",
         "//apex_available:anyapex",
     ],
+    visibility: ["//system/linkerconfig"],
 }
 
 python_library_host {