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/cadence/xtfpga/xtfpga.c b/board/cadence/xtfpga/xtfpga.c
index bdda931..2566116 100644
--- a/board/cadence/xtfpga/xtfpga.c
+++ b/board/cadence/xtfpga/xtfpga.c
@@ -8,6 +8,7 @@
 #include <command.h>
 #include <dm.h>
 #include <dm/platform_data/net_ethoc.h>
+#include <env.h>
 #include <linux/ctype.h>
 #include <linux/string.h>
 #include <linux/stringify.h>