invoke singletons() function to satisfy compiler

zig-ecs/tests/registry_test.zig

@@ -82,12 +82,12 @@ test "singletons" {
     defer reg.deinit();
 
     var pos = Position{ .x = 5, .y = 5 };
-    reg.singletons.add(pos);
-    try std.testing.expect(reg.singletons.has(Position));
-    try std.testing.expectEqual(reg.singletons.get(Position).*, pos);
+    reg.singletons().add(pos);
+    try std.testing.expect(reg.singletons().has(Position));
+    try std.testing.expectEqual(reg.singletons().get(Position).*, pos);
 
-    reg.singletons.remove(Position);
-    try std.testing.expect(!reg.singletons.has(Position));
+    reg.singletons().remove(Position);
+    try std.testing.expect(!reg.singletons().has(Position));
 }
 
 test "destroy" {