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>

examples/api/demo.c

@@ -6,6 +6,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <linux/types.h>
 #include <api_public.h>