Styx/RealTimeClockLib: hide LibRtcVirtualNotifyEvent

The RealTimeClockLib class header in edk2 mistakenly declares a function
called LibRtcVirtualNotifyEvent(). No component ever calls this function
crossing module boundaries; all RealTimeClockLib instances in edk2 and
edk2-platforms are supposed to register (and do register) their
SetVirtualAddressMap() notification functions.

Rename LibRtcVirtualNotifyEvent() to VirtualNotifyEvent(), and make it
static, in preparation for removing the LibRtcVirtualNotifyEvent()
declaration from the lib class header later. Also hoist the function
definition above the reference.

Build-tested only (with "OverdriveBoard.dsc", "CelloBoard.dsc",
"Overdrive1000Board.dsc").

Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=4564


Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>