libsidplayfp: set compiler.cxx_standard 2020

[aeiouaeiouaeiouaeiouaeiouaeiou]

Tested on

macOS 10.8.5 12F2560 x86_64
Xcode 5.1.1 5B1008

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[macportsbot]

Notifying maintainers:
@mohd-akram for port libsidplayfp.

[mohd-akram]

This is surprising. It seems like this was unintentional since this library tries to maintain C++11 support. I wonder if wrapping #include <version> in #ifdef HAVE_CXX20 is enough to resolve the build failure. I've opened libsidplayfp/libsidplayfp#214 in the meantime.