Update CTurbSSTSolver.cpp to fix a bug of calculate MuT.

[ShiheJia]

Proposed Changes

When calculating the turb viscosity MuT for the SST 2003 version, the variable StrainMag is needed. However, in the current code, we use the wrong CVariable class to get the StrainMag so that we only get 0. The turb variable class should be replaced by the flow variable class.

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