manager: fd must be int, not char

This should fix the crash reported by Dan Horak on s390x which
does not have VTs.

src/manager.c

@@ -127,7 +127,7 @@ static int manager_setup_notify(Manager *m) {
 }
 
 static int enable_special_signals(Manager *m) {
-        char fd;
+        int fd;
 
         assert(m);