[microsoft-signalr] Fix build error on Linux

[NancyLi1013]

Describe the pull request

• What does your PR fix? Fixes [microsoft-signalr] Build failure #14860

When building on Linux, it will fail due to this error:

error: cannot use typeid with -fno-rtti

There are two reasons to fix this problem:

1. -fno-rtti is only for non windows Platform.
2. typeid is an RTTI-feature. So turning off RTTI (-fno-rtti) also disables features like typeid.

-fno-rtti should not be added here https://github.com/aspnet/SignalR-Client-Cpp/blob/master/src/signalrclient/CMakeLists.txt#L63.

So I make a patch to remove these codes.

    # GCC on OSX has a bug with exceptions and no-rtti that can cause crashes
    if(NOT APPLE)
        target_compile_options(signalrclient PRIVATE -fno-rtti)
    endif()

Please see the references:

[1] https://scc.ustc.edu.cn/zlsc/tc4600/intel/2017.0.098/compiler_c/common/core/GUID-BA6071E9-2409-4118-84B5-8E1ABE0C2B72.html
[2] https://stackoverflow.com/questions/8723702/error-cannot-use-typeid-with-fno-rtti

Note: No need to test features.

[BillyONeal]

Thanks for your contribution!