simple-ipc: move definition of ipc_active_state outside of ifdef

simple-ipc: move definition of ipc_active_state outside of ifdef

Move the declartion of the `enum ipc_active_state` type outside of
the SUPPORTS_SIMPLE_IPC ifdef.

A later commit will introduce the `fsmonitor_ipc__*()` API and stub in
a "mock" implementation that requires this enum in some function
signatures.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>