build: keep DirectLinkUpload Rule

app/src/main/java/io/legado/app/help/DirectLinkUpload.kt

@@ -66,10 +66,11 @@ object DirectLinkUpload {
         return getRule()?.summary
     }
 
+    @keep
     data class Rule(
         var uploadUrl: String,
         var downloadUrlRule: String,
         var summary: String?
     )
 
-}
+}