security/softether5: fix wrong CMAKE_BOOL_ON
PR: 268294
No macro with this name is checked by USES=cmake. The correct
one would be CMAKE_ON, but that one is not evaluated again after
bsd.port.pre.mk. Just directly add -DSKIP_CPU_FEATURES=1 to
CMAKE_ARGS to work around this shortcoming.
(cherry picked from commit e1e189c2117ca7337a6133cb45523c249b6a7cd3)