Fix Clang -Winconsistent-missing-override warning

[bitsawer]

Adds missing override and final qualifiers to VulkanContextWindows::_get_platform_surface_extension(), similar to other VulkanContext implementations. Missing these causes a build error when building with dev_mode enabled (warnings as errors):

In file included from platform\windows\godot_windows.cpp:31:
In file included from platform\windows/os_windows.h:53:
platform\windows/vulkan_context_win.h:42:22: error: '_get_platform_surface_extension' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
        virtual const char *_get_platform_surface_extension() const;
                            ^
./drivers/vulkan/vulkan_context.h:261:22: note: overridden virtual function is here
        virtual const char *_get_platform_surface_extension() const { return nullptr; }
                            ^
1 error generated.

[akien-mga]

Thanks!