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/tools/env/fw_env.h b/tools/env/fw_env.h
index b250e2f..3d2b457 100644
--- a/tools/env/fw_env.h
+++ b/tools/env/fw_env.h
@@ -4,6 +4,7 @@
  * Wolfgang Denk, DENX Software Engineering, wd@denx.de.
  */
 
+#include <env.h>
 #include <stdint.h>
 
 /*