Add missing include cmath in port.cpp

[quanterium]

While attempting to build HEAD on CentOS 8.1 I encountered the following error:

port.cpp: In member function ‘void Port::setAveragePacketRate(double)’:
port.cpp:192:18: error: ‘isnan’ is not a member of ‘std’
         if (std::isnan(rate))
                  ^~~~~
port.cpp: In member function ‘void Port::setAverageBitRate(double)’:
port.cpp:271:18: error: ‘isnan’ is not a member of ‘std’
         if (std::isnan(rate))
                  ^~~~~

Adding #include <cmath> to client/port.cpp allowed ostinato to compile successfully.

[pstavirs]

Thanks for the PR @quanterium!