xen/timer: rename "timer.c" to "xen_timer.c"
The better to avoid conflicting with other files given how "timer.c"
isn't a very unique name. ARM and RISC-V both have timer.c files.
Reviewed by: royger, imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/609
Differential Revision: https://reviews.freebsd.org/D36258