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>

tools/env/fw_env_main.c

@@ -26,6 +26,7 @@
  * of AES key), eg. '-a aabbccddeeff00112233445566778899'.
  */
 
+#include <env.h>
 #include <fcntl.h>
 #include <getopt.h>
 #include <stdio.h>