env: Move env_get_f() to env.h

Move this function over to the new header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/cmd/mtdparts.c b/cmd/mtdparts.c
index f7ed1a0..46155ca 100644
--- a/cmd/mtdparts.c
+++ b/cmd/mtdparts.c
@@ -72,6 +72,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <env.h>
 #include <malloc.h>
 #include <jffs2/load_kernel.h>
 #include <linux/list.h>