isnan is ambiguous

[bigfooted]

reported from here:
https://www.cfd-online.com/Forums/su2-installation/261261-compile-problem-su2-8-1-0-a.html#post885149

/SU2_CFD/src/output/CFlowCompOutput.cpp:776:58: error: call of overloaded ‘isnan(const su2double&)’ is ambiguous

I think this is due to the use of older compilers as reported here:

#1112

[joshkellyjak]

This is in the turbo output, will take a look tonight.

[joshkellyjak]

Got distracted. Resolved in #2577

[bigfooted]

Thanks!