Fix videolan/LibVLCSharp#537

Fixes Managed Debugging Assistant 'PInvokeStackImbalance'
A call to PInvoke function 'LibVLCSharp.WPF!LibVLCSharp.WPF.User32Wrapper::CreateWindowEx' has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature. Check that the calling convention and parameters of the PInvoke signature match the target unmanaged signature.

Close videolan/LibVLCSharp#529

this adds a build check for CI

multi targeting new and legacy TFMs is not supported on mac,
so we don't build net6 TFMs on mac and with our samples having project references,
this fails the samples build.

net6.0 warns that this is always false...