make warnings start with "htmlroff: ".

Reference: /n/sources/patch/applied/htmlroff-warn
Date: Sat Jan 28 12:40:42 CET 2006

--- /sys/src/cmd/htmlroff/util.c	Sat Jan 28 12:40:28 2006
+++ /sys/src/cmd/htmlroff/util.c	Sat Jan 28 12:40:26 2006
@@ -78,7 +78,7 @@
 {
 	va_list arg;
 	
-	fprint(2, "%L: ");
+	fprint(2, "htmlroff: %L: ");
 	va_start(arg, fmt);
 	vfprint(2, fmt, arg);
 	va_end(arg);