common: Drop bootstage.h from common header

Move this fairly uncommon header out of the common header.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/post/post.c b/post/post.c
index 696a60f..0f977fe 100644
--- a/post/post.c
+++ b/post/post.c
@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <bootstage.h>
 #include <env.h>
 #include <malloc.h>
 #include <stdio_dev.h>