Add a dummy WaitStatus.TrapCause for Darwin to unbreak the build.

R=gri
APPROVED=gri
DELTA=5  (5 added, 0 deleted, 0 changed)
OCL=31673
CL=31673

src/pkg/syscall/syscall_darwin.go

@@ -135,6 +135,11 @@ func (w WaitStatus) StopSignal() int {
 	return int(w >> shift) & 0xFF;
 }
 
+func (w WaitStatus) TrapCause() int {
+	// Darwin doesn't have trap causes
+	return -1;
+}
+
 //sys	wait4(pid int, wstatus *_C_int, options int, rusage *Rusage) (wpid int, errno int)
 func Wait4(pid int, wstatus *WaitStatus, options int, rusage *Rusage) (wpid int, errno int) {
 	var status _C_int;