roken: move Heimdal vis.h extensions to separate header

vis.h is not built on platforms (such as macOS) that already have
that header, which resulted in prototypes for Heimdal-specific
vis.h extensions being absent.

Move those prototypes to a separate header, vis-extras.h, which
must be explicitly included in order to use the Heimdal extensions.

lib/hx509/print.c

@@ -33,6 +33,7 @@
 
 #include "hx_locl.h"
 #include <vis.h>
+#include <vis-extras.h>
 
 /**
  * @page page_print Hx509 printing functions