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/cmd/pxe.c b/cmd/pxe.c
index 1dd0a74..2059975 100644
--- a/cmd/pxe.c
+++ b/cmd/pxe.c
@@ -6,6 +6,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <env.h>
 #include <malloc.h>
 #include <mapmem.h>
 #include <lcd.h>