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/asn1/gen_template.c

@@ -41,6 +41,7 @@
 
 #include "gen_locl.h"
 #include <vis.h>
+#include <vis-extras.h>
 
 static const char *symbol_name(const char *, const Type *);
 static void generate_template_type(const char *, const char **, const char *, const char *, const char *,