syscall: fix prototype of Fchflags (API change)
API change, but the old API is obviously wrong.

R=golang-dev, iant, r, rsc
CC=golang-dev
https://golang.org/cl/9157044
diff --git a/src/pkg/syscall/syscall_freebsd.go b/src/pkg/syscall/syscall_freebsd.go
index c7ffe22..9fbcc48 100644
--- a/src/pkg/syscall/syscall_freebsd.go
+++ b/src/pkg/syscall/syscall_freebsd.go
@@ -118,7 +118,7 @@
 //sysnb	Dup2(from int, to int) (err error)
 //sys	Exit(code int)
 //sys	Fchdir(fd int) (err error)
-//sys	Fchflags(path string, flags int) (err error)
+//sys	Fchflags(fd int, flags int) (err error)
 //sys	Fchmod(fd int, mode uint32) (err error)
 //sys	Fchown(fd int, uid int, gid int) (err error)
 //sys	Flock(fd int, how int) (err error)