libflashprog: Mark some internal structs as @private

Change-Id: Idcf810b7d7c2dbde7587def04eb693d171836fe2
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/514
diff --git a/layout.c b/layout.c
index 24e25e7..26b6273 100644
--- a/layout.c
+++ b/layout.c
@@ -24,10 +24,12 @@
 #include "programmer.h"
 #include "layout.h"
 
+/** @private */
 struct flashprog_layout {
 	struct romentry *head;
 };
 
+/** @private */
 struct layout_include_args {
 	char *name;
 	struct layout_include_args *next;