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/gateworks/gw_ventana/gw_ventana.c b/board/gateworks/gw_ventana/gw_ventana.c
index 92edc10..3c40a4a 100644
--- a/board/gateworks/gw_ventana/gw_ventana.c
+++ b/board/gateworks/gw_ventana/gw_ventana.c
@@ -21,6 +21,7 @@
 #include <asm/setup.h>
 #include <dm.h>
 #include <dm/platform_data/serial_mxc.h>
+#include <env.h>
 #include <environment.h>
 #include <hwconfig.h>
 #include <i2c.h>