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/zip/cmd/Android.bp b/zip/cmd/Android.bp
index 43bf232..16c3f69 100644
--- a/zip/cmd/Android.bp
+++ b/zip/cmd/Android.bp
@@ -24,4 +24,6 @@
     srcs: [
         "main.go",
     ],
+    // Used by genrules
+    visibility: ["//visibility:public"],
 }