security/softether5: conditionalize dependence on devel/cpu_features

devel/cpu_features is not available on some architectures after r496118.
Furturemore, upstream doesn't require cpu_features on some architectures
such as armv7l, aarch64, s390x. Add CFLAGS=-DSKIP_CPU_FEATURES on
architectures devel/cpu_features is not available.

Tested no-harm on amd64.