syscalls: annotate Sendfile() for race detector
Fixes #4306.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/6816054
diff --git a/src/pkg/syscall/syscall_openbsd.go b/src/pkg/syscall/syscall_openbsd.go
index 8f2b085..a146dca 100644
--- a/src/pkg/syscall/syscall_openbsd.go
+++ b/src/pkg/syscall/syscall_openbsd.go
@@ -91,7 +91,7 @@
 }
 
 // TODO
-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) {
 	return -1, ENOSYS
 }