i965: Silence unused variable warning on non-debug builds.

Fixes this GCC warning.
brw_vs.c: In function 'do_vs_prog':
brw_vs.c:46: warning: unused variable 'ctx'

src/mesa/drivers/dri/i965/brw_vs.c

@@ -96,6 +96,7 @@ static void do_vs_prog( struct brw_context *brw,
 	  sizeof(c.prog_data));
    assert(ctx->Const.VertexProgram.MaxNativeParameters ==
 	  ARRAY_SIZE(c.constant_map));
+   (void) ctx;
 
    aux_size = sizeof(c.prog_data);
    if (c.vp->use_const_buffer)