extensions/khr: Re-add erroneously removed `mod acceleration_structur…

…e` (#893)

While rush-rebasing 5 extensions in a row #889 seems to have ended
up deleting the first line of `mod.rs`, which was defining the
`acceleration_structure` module.  Restore that.

ash/src/extensions/khr/mod.rs

@@ -1,3 +1,4 @@
+pub mod acceleration_structure;
 pub mod android_surface;
 pub mod buffer_device_address;
 pub mod calibrated_timestamps;