Disable MSVC posix security warnings in the project file.

By default Visual Studio warns about various C stdlib calls,
recommending non-portable replacements instead. We disable
this warning in a number of places in the source.

Since they're specific to the Visual Studio build, it's better
to disable them just in the project files where they always
apply to the correct toolchain.

I have only added the disable setting to project files which
need it currently: opus, test_opus_encode, and test_opus_decode.