syscall: fix build

I missed the API change in the last FreeBSD CL, but the tool caught it.

TBR=bradfitz
CC=golang-dev
https://golang.org/cl/6331063

src/pkg/syscall/syscall_freebsd.go

@@ -89,9 +89,9 @@ func Pipe(p []int) (err error) {
 	return
 }
 
-func Sendfile(outfd int, infd int, offset int64, count int) (written int, err error) {
+func Sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
 	var writtenOut uint64 = 0
-	_, _, e1 := Syscall9(SYS_SENDFILE, uintptr(infd), uintptr(outfd), uintptr(offset), uintptr(count), 0, uintptr(unsafe.Pointer(&writtenOut)), 0, 0, 0)
+	_, _, e1 := Syscall9(SYS_SENDFILE, uintptr(infd), uintptr(outfd), uintptr(*offset), uintptr(count), 0, uintptr(unsafe.Pointer(&writtenOut)), 0, 0, 0)
 
 	written = int(writtenOut)