doxygen: add newline after \file

While \file doesn't need an argument, it can't have another doxy
command after it.

source/blender/draw/intern/draw_manager.c

@@ -16,7 +16,8 @@
  * Copyright 2016, Blender Foundation.
  */
 
-/** \file \ingroup draw
+/** \file
+ * \ingroup draw
  */
 
 #include <stdio.h>