set the malloc tag.

Reference: /n/patches.lsub.org/patch/strdup
Date: Sun Apr 15 12:56:24 CES 2012
Signed-off-by: nemo@lsub.org

--- /sys/src/libc/port/strdup.c	Thu Apr 12 12:26:18 2012
+++ /sys/src/libc/port/strdup.c	Wed Jul  6 21:58:20 2011
@@ -9,6 +9,7 @@
 	ns = malloc(strlen(s) + 1);
 	if(ns == 0)
 		return 0;
+	setmalloctag(ns, getcallerpc(&s));
 
 	return strcpy(ns, s);
 }