Fix requirement on libcanberra
I wanted to have it break at depsolve time, so that:
1. the user would have a more explicit error message when
installing / updating
2. I'd get an early notice in the broken deps report
It seems like it's not possible to depend on the precise
soname without causing arch-specific issues (the soname
is arch-dependent, in an arch-dependent folder, but this
package is noarch), though.