L1TopoAlgorithms: Fix clang warning.

 - fabs(int).  Use std::abs instead.
 

Trigger/TrigT1/L1Topo/L1TopoAlgorithms/Root/DeltaRApproxBoxCutIncl2.cxx

 /*
-  Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
 */
 /*********************************
  * DeltaRApproxBoxCutIncl2.cpp
@@ -33,7 +33,7 @@ namespace {
 
    unsigned int
    calcDeltaEta(const TCS::GenericTOB* tob1, const TCS::GenericTOB* tob2) {
-      double deta = fabs( tob1->eta() - tob2->eta() );
+      double deta = std::abs( tob1->eta() - tob2->eta() );
       return deta;
    }
 }