st/python: add usage parameter to pipe_buffer_create

This is a follow-up to commit eafb7f234d.

Fixes Linux SCons build.

src/gallium/state_trackers/python/p_context.i

@@ -350,6 +350,7 @@ struct st_context {
 
       vbuf = pipe_buffer_create(screen,
                                 PIPE_BIND_VERTEX_BUFFER, 
+                                PIPE_USAGE_STATIC,
                                 size);
       if(!vbuf)
          goto error1;

src/gallium/state_trackers/python/p_device.i

@@ -134,7 +134,7 @@ struct st_device {
    }
 
    struct pipe_resource *
-   buffer_create(unsigned size, unsigned bind = 0) {
-      return pipe_buffer_create($self->screen, bind, size);
+   buffer_create(unsigned size, unsigned usage, unsigned bind = 0) {
+      return pipe_buffer_create($self->screen, bind, usage, size);
    }
 };