liebherr: Include env.h to permit reading the environment

This file reads from the environment but does not include the correct
header. Update it.

Signed-off-by: Simon Glass <sjg@chromium.org>

board/liebherr/xea/xea.c

@@ -13,6 +13,7 @@
  *
  */
 
+#include <env.h>
 #include <fdt_support.h>
 #include <init.h>
 #include <log.h>