Restore the ''import "unsafe'' removed in 36daedf3.

This was causing build failures with tools dependent on Flatbuffers in Go.
E.g.
go/src/github.com/google/flatbuffers/go/sizes.go:50: undefined: unsafe in unsafe.Pointer

go/sizes.go

 package flatbuffers
 
+import (
+        "unsafe"
+)
+
 const (
 	// See http://golang.org/ref/spec#Numeric_types