env: Move env_set() to env.h

Move env_set() over to the new header file.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/xilinx/zynq/board.c b/board/xilinx/zynq/board.c
index 6857f2c..35191b2 100644
--- a/board/xilinx/zynq/board.c
+++ b/board/xilinx/zynq/board.c
@@ -6,6 +6,7 @@
 
 #include <common.h>
 #include <dm/uclass.h>
+#include <env.h>
 #include <fdtdec.h>
 #include <fpga.h>
 #include <malloc.h>