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/extra.c

@@ -35,7 +35,8 @@
 
 #include "der_locl.h"
 #include "heim_asn1.h"
-#include "vis.h"
+#include <vis.h>
+#include <vis-extras.h>
 
 RCSID("$Id$");
 

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 *,

lib/asn1/template.c

@@ -36,6 +36,7 @@
 #include "der_locl.h"
 #include <com_err.h>
 #include <vis.h>
+#include <vis-extras.h>
 
 struct asn1_type_func asn1_template_prim[A1T_NUM_ENTRY] = {
 #define el(name, type) {				\