add utfnlen declaration
chan.c calls it

Reference: /n/sources/patch/applied/utfnlen_missing
Date: Tue Sep  6 14:16:40 CES 2005

--- /sys/src/9/port/lib.h	Tue Sep  6 14:10:45 2005
+++ /sys/src/9/port/lib.h	Tue Sep  6 14:10:41 2005
@@ -43,6 +43,7 @@
 extern	int	chartorune(Rune*, char*);
 extern	char*	utfrune(char*, long);
 extern	int	utflen(char*);
+extern	int	utfnlen(char*, long);
 extern	int	runelen(long);
 
 extern	int	abs(int);