fix a botch (noticed and reported by sl.  thanks!) in upas config
must use a constant initializer

Reference: /n/atom/patch/applied/upascfgbotch
Date: Tue Apr 21 20:56:24 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/upas/common/config.c	Tue Apr 21 20:56:00 2015
+++ /sys/src/cmd/upas/common/config.c	Tue Apr 21 20:56:01 2015
@@ -1,7 +1,7 @@
 #include "common.h"
 
 char *MAILROOT = "/mail";
-char *SPOOL	= MAILROOT;
+char *SPOOL	= "/mail";
 char *UPASLOG	= "/sys/log";
 char *UPASLIB	= "/mail/lib";
 char *UPASBIN	= "/bin/upas";