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/roken/vis.c

@@ -396,6 +396,8 @@ rk_strsvisx(char *dst, const char *csrc, size_t len, int flag, const char *extra
  * buffer as needed.  Based on OpenBSD's stravis().
  */
 
+#include <vis-extras.h>
+
 ROKEN_LIB_FUNCTION int ROKEN_LIB_CALL
 rk_strasvis(char **out, const char *csrc, int flag, const char *extra)
 {