[spinel] fix outdated comment (#3814)
[spinel] fix outdated comment (#3814)

`OPENTHREAD_ENABLE_POSIX_APP` was merged into
`OPENTHREAD_PLATFORM_POSIX_APP`.
librasungirl/openthread,turon/openthread,chshu/openthread,abtink/openthread,lanyuwen/openthread,jwhui/openthread,lanyuwen/openthread,chshu/openthread,srickardti/openthread,bukepo/openthread,abtink/openthread,lanyuwen/openthread,srickardti/openthread,bukepo/openthread,librasungirl/openthread,abtink/openthread,librasungirl/openthread,jwhui/openthread,librasungirl/openthread,abtink/openthread,srickardti/openthread,turon/openthread,bukepo/openthread,openthread/openthread,lanyuwen/openthread,chshu/openthread,srickardti/openthread,openthread/openthread,turon/openthread,jwhui/openthread,jwhui/openthread,turon/openthread,chshu/openthread,openthread/openthread,chshu/openthread,chshu/openthread,bukepo/openthread,lanyuwen/openthread,openthread/openthread,lanyuwen/openthread