Fix build of C++ code with clang >= 16 and -Wundef
/path/to/wayland-util.h:73:5: error: 'STDC_VERSION' is not defined, evaluates to 0 [-Werror,-Wundef]
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/path/to/wayland-util.h:73:5: error: 'STDC_VERSION' is not defined, evaluates to 0 [-Werror,-Wundef]