target-helpers: If neither softpipe or llvmpipe is used just return the screen

So the targets can drop the sw_wrapper winsys when no sw driver is being used.

Signed-off-by: Jakob Bornecrantz <jakob@vmware.com>
Reviewed-by Brian Paul <brianp@vmware.com>

src/gallium/auxiliary/target-helpers/inline_wrapper_sw_helper.h

 static INLINE struct pipe_screen *
 sw_screen_wrap(struct pipe_screen *screen)
 {
+#if defined(GALLIUM_SOFTPIPE) || defined(GALLIUM_LLVMPIPE)
    struct sw_winsys *sws;
    struct pipe_screen *sw_screen = NULL;
    const char *driver;
 err_winsys:
    return wrapper_sw_winsys_dewrap_pipe_screen(sws);
 err:
+#endif
    return screen;
 }