cpu-kvm: Add missing 'override' keyword

cpu-kvm: Add missing 'override' keyword

clang requires all functions that override a member function to be
masked by the 'override' keyword. The missing 'override' in
timer.hh causes compiling issues while compiling gem5 with clang.
This commit adds the missing keyword.

Jira: https://gem5.atlassian.net/browse/GEM5-724

Change-Id: I3b5c7af666927b079a785803c8bb4869180ff777
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32095
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>