Fix build on Plan 9

mmap_plan9.go

@@ -0,0 +1,27 @@
+// Copyright 2020 Evan Shaw. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mmap
+
+import "syscall"
+
+func mmap(len int, inprot, inflags, fd uintptr, off int64) ([]byte, error) {
+	return nil, syscall.EPLAN9
+}
+
+func (m MMap) flush() error {
+	return syscall.EPLAN9
+}
+
+func (m MMap) lock() error {
+	return syscall.EPLAN9
+}
+
+func (m MMap) unlock() error {
+	return syscall.EPLAN9
+}
+
+func (m MMap) unmap() error {
+	return syscall.EPLAN9
+}