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 <a0aa8449aacb303edcb25d6bbe69da9124f010fc@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32095
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5