vl/vl_winsys: Add missing include for function declaration

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/12150
Fixes: 2548471ca1 ("vl/vl_win32_screen_create_from_d3d12_device: Allow winsys to be injected")
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32559>

src/gallium/auxiliary/vl/vl_winsys.h

@@ -46,6 +46,7 @@ extern "C" {
 #endif
 #include "pipe/p_defines.h"
 #include "util/format/u_formats.h"
+#include "frontend/sw_winsys.h"
 
 struct pipe_screen;
 struct pipe_surface;