fix the o2-linter error

Use std::fabs for absolute value of eta

Author: wenyaCern

PWGCF/Flow/Tasks/flowFlucGfwPp.cxx

@@ -1015,14 +1015,14 @@ struct FlowFlucGfwPp {
         registry.fill(HIST("qvecQA/ChTracks"), trk.pt(), trk.eta(), trk.phi());
       }
 
-      if (trk.eta() > 0 && fabs(trk.eta()) < cfgQnTrkAbsEtaMax) {
+      if (trk.eta() > 0 && std::fabs(trk.eta()) < cfgQnTrkAbsEtaMax) {
         // In qVectorsTable this branch is additionally guarded by useDetector["QvectorTPCposs"] || useDetector["QvectorBPoss"].
         // Here TPCpos is always computed because the downstream ESE selector can require it.
         qvec.qVectTPCPos[0] += trk.pt() * std::cos(trk.phi() * nMode);
         qvec.qVectTPCPos[1] += trk.pt() * std::sin(trk.phi() * nMode);
         qvec.trkTPCPosLabel.push_back(trk.globalIndex());
         qvec.nTrkTPCPos++;
-      } else if (trk.eta() < 0 && fabs(trk.eta()) < cfgQnTrkAbsEtaMax) {
+      } else if (trk.eta() < 0 && std::fabs(trk.eta()) < cfgQnTrkAbsEtaMax) {
         // In qVectorsTable this branch is additionally guarded by useDetector["QvectorTPCnegs"] || useDetector["QvectorBNegs"].
         // Here TPCneg is always computed because the downstream ESE selector can require it.
         qvec.qVectTPCNeg[0] += trk.pt() * std::cos(trk.phi() * nMode);