Fix Ruby 4.0 warning: use require_relative for internal files - #148

Ruby 4.0 removed `benchmark` from default gems. When benchmark-ips
uses `require 'benchmark/timing'`, Ruby's gem activation system
sees the `benchmark/` prefix and warns that the `benchmark` gem is
not a default gem, even though all these files belong to
benchmark-ips.

Using `require_relative` bypasses gem activation entirely, resolving
the warning:

  benchmark/timing is found in benchmark, which is not part of the
  default gems since Ruby 4.0.0.