Fix non sized warning for ViaXml

[tsurai]

Tiny PR fixing a warning in advance before it becomes a HARD ERROR in the next release.

src/lib.rs:117:5: 117:59 warning: the trait `core::marker::Sized` is not implemented for the type `Self` [E0277]
src/lib.rs:117     fn from_xml(elem: Element) -> Result<Self, ReadError>;
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/lib.rs:117:5: 117:59 help: run `rustc --explain E0277` to see a detailed explanation
src/lib.rs:117:5: 117:59 note: `Self` does not have a constant size known at compile-time
src/lib.rs:117:5: 117:59 note: this warning results from recent bug fixes and clarifications; it will become a HARD ERROR in the next release. See RFC 1214 for details.
src/lib.rs:117     fn from_xml(elem: Element) -> Result<Self, ReadError>;
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[frewsxcv]

Thanks!

[frewsxcv]

Just published version 0.2.1 that includes this change. Thanks again!