Build with -Wmissing-prototypes flags vPortYieldFromISR() in the Posix port. (#409)

Build with -Wmissing-prototypes flags vPortYieldFromISR() in the Posix port. (#409)

There's already a portYIELD_FROM_ISR() macro that calls vPortYield() which wraps the FromISR code.
It doesn't appear that vPortYieldFromISR() is intended to be publicly accessible in this port so
I've marked it as private to silence the warning.

event_create() also got flagged due to missing void in prototype.

Co-authored-by: Gaurav-Aggarwal-AWS <33462878+aggarg@users.noreply.github.com>