GOnce initialization doesn't work in cl.exe

		private static GLib.Once<ThreadPool<TaskData<G>>> async_pool;

(in line 72 of gee/task.vala from libgee) compiles to

static GOnce gee_task_data_async_pool = (GOnce) G_ONCE_INIT;

using Vala 0.56.9.

This fails to compile using cl.exe (Microsoft (R) C/C++ Optimizing Compiler Version 19.36.32534 for x64):

subprojects/libgee-0.20.6/gee/gee-0.8-2.6.1.dll.p/task.c(59): error C2099: initializer is not a constant

Removing the (GOnce) cast fixes the issue.