syscall: add Flock on Linux

Fixes #2069.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4703048

src/pkg/syscall/zsyscall_linux_amd64.go

diff --git a/src/pkg/syscall/zsyscall_linux_amd64.go b/src/pkg/syscall/zsyscall_linux_amd64.go
index fa20ff5..999ed64 100644
--- a/src/pkg/syscall/zsyscall_linux_amd64.go
+++ b/src/pkg/syscall/zsyscall_linux_amd64.go
@@ -277,6 +277,14 @@
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func Flock(fd int, how int) (errno int) {
+	_, _, e1 := Syscall(SYS_FLOCK, uintptr(fd), uintptr(how), 0)
+	errno = int(e1)
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func Fsync(fd int) (errno int) {
 	_, _, e1 := Syscall(SYS_FSYNC, uintptr(fd), 0, 0)
 	errno = int(e1)