Bad auto_serialize code

[jesse99]

As part of #4041 tried to get Tm auto serializing, but was getting

/Volumes/SSD/rust/src/libstd/time.rs:138:8: 138:11 error: binding `__v0` should use ref or copy mode
/Volumes/SSD/rust/src/libstd/time.rs:138     Tm_(Tm_)
                                                 ^~~
/Volumes/SSD/rust/src/libstd/time.rs:138:8: 138:11 error: binding `__v0` should use ref or copy mode
/Volumes/SSD/rust/src/libstd/time.rs:138     Tm_(Tm_)
                                                 ^~~
error: aborting due to 2 previous errors