Extra define needed for pi constant for RadarChart on Windows

[matty0ung]

Fix for #538

I don't have a Windows build set up on my machine, so it would be great if someone who has could quickly check this compiles as expected.

[Lubart]

Compiling on Windows 10 under Visual Studio 2022 still getting error. Please see below.
C++ Language Standard used: ISO C++17 Standard (/std:c++17).
3>RadarChart.cpp
3>D:\Git\brewtarget\src\RadarChart.cpp(50,40): error C2065: 'M_PI': undeclared identifier
3>D:\Git\brewtarget\src\RadarChart.cpp(50,17): error C2737: '`anonymous-namespace'::RadiansInACircle': const object must be initialized

[matty0ung]

Ah, interesting. I think there is a straightforward fix now we're on C++20. Will have a look soon.