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/lib/efi_loader/efi_variable.c b/lib/efi_loader/efi_variable.c
index 889a7f2..8a0e0db 100644
--- a/lib/efi_loader/efi_variable.c
+++ b/lib/efi_loader/efi_variable.c
@@ -5,6 +5,7 @@
  *  Copyright (c) 2017 Rob Clark
  */
 
+#include <env.h>
 #include <malloc.h>
 #include <charset.h>
 #include <efi_loader.h>