Export XInput2 from x11-dl crate

x11/src/lib.rs and x11-dl/src/lib.rs are most duplicates
of each other. It would be preferable to combine them into
a single file. This could be done by building
src/lib.rs with different config attributes (eg. cfg=dynamic) but
at present it doesn't look like it is possible
to have different exports depending on #[cfg(attribute)]

See rust-lang/rust#25544

Author: Robert Knight

x11-dl/src/lib.rs

@@ -23,6 +23,7 @@ pub mod xf86vmode;
 pub mod xfixes;
 pub mod xinerama;
 pub mod xinput;
+pub mod xinput2;
 pub mod xlib;
 pub mod xmu;
 pub mod xrender;