llvmpipe: Change return type of declaration to match code.

Commit 2142108e0e changed the return type
of llvmpipe_get_texture_image_address function but didn't change the
declaration.

Fixes build.

src/gallium/drivers/llvmpipe/lp_texture.h

@@ -189,7 +189,7 @@ void *
 llvmpipe_resource_data(struct pipe_resource *resource);
 
 
-void *
+ubyte *
 llvmpipe_get_texture_image_address(struct llvmpipe_resource *lpr,
                                     unsigned face_slice, unsigned level,
                                     enum lp_texture_layout layout);