CxxUtils, TrigConfBase: add missing includes visible in dev4 builds

Johannes Elmsheuserrequested to merge
elmsheus/athena:dev4_20210907 into master

There are 2 missing includes failures visible in the latest dev4 nightly builds:

  • Add missing cmath include in StringUtils.cxx
  • Add missing sstream include in TrigDBConnectionConfig.cxx

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