add surrogate to mkfile

Reference: /n/atom/patch/applied/surrogatemk
Date: Thu May 15 02:37:41 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libc/port/mkfile	Thu May 15 02:37:33 2014
+++ /sys/src/libc/port/mkfile	Thu May 15 02:37:35 2014
@@ -103,6 +103,7 @@
 	strtoll.c\
 	strtoul.c\
 	strtoull.c\
+	surrogatetorune.c\
 	tan.c\
 	tanh.c\
 	tobaserune.c\