codecgen: handle struct fields of type *time.Time correctly

[ugorji]

If a type has a variable of type *time.Time, the generated code does not compile.

We get errors like those seen in gyuho/etcd@1743d96#commitcomment-28455954

./keys.generated.go:1939:23: cannot use x.Expiration (type *time.Time) as type time.Time in argument to r.encDriver.EncodeTime
./keys.generated.go:1959:23: cannot use x.Expiration (type *time.Time) as type time.Time in argument to r.encDriver.EncodeTime

These are captured in

• go get github.com/cores/etcd/client fails on generated code etcd-io/etcd#8715
• Please upgrade to ugorji/go/codec v1.1.1 containing 32-bit fixes (when available) etcd-io/etcd#9447