loader: Fix cast warning in wsi.c
loader: Fix cast warning in wsi.c

In 32-bits, this was downcasting a VkSurfaceKHR and comparing it
with NULL. Since VkSurfaceKHR is defined as a uint64_t, it's better
to cast the NULL to a VkSurfaceKHR to avoid a warning.

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools