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/include/env_flags.h b/include/env_flags.h
index 23744e3..f230643 100644
--- a/include/env_flags.h
+++ b/include/env_flags.h
@@ -146,6 +146,7 @@
 
 #else /* !USE_HOSTCC */
 
+#include <env.h>
 #include <search.h>
 
 /*