Fix WindowProVider.fn_getX11Window def

This was missed in 3ae4a71

Partial fix for zig-gamedev/zig-gamedev#753

Author: hazeycode

src/zgpu.zig

@@ -24,7 +24,7 @@ pub const WindowProvider = struct {
     fn_getFramebufferSize: *const fn (window: *const anyopaque) [2]u32,
     fn_getWin32Window: *const fn (window: *const anyopaque) callconv(.c) *anyopaque = undefined,
     fn_getX11Display: *const fn () callconv(.c) *anyopaque = undefined,
-    fn_getX11Window: *const fn (window: *const anyopaque) u32 = undefined,
+    fn_getX11Window: *const fn (window: *const anyopaque) callconv(.c) u32 = undefined,
     fn_getWaylandDisplay: ?*const fn () callconv(.c) *anyopaque = null,
     fn_getWaylandSurface: ?*const fn (window: *const anyopaque) callconv(.c) *anyopaque = null,
     fn_getCocoaWindow: *const fn (window: *const anyopaque) callconv(.c) ?*anyopaque = undefined,