lib: os: mpsc_pbuf: fix potential semaphore wait forever

One thread calls mpsc_pbuf_alloc to produce data, which invokes
add_skip_item and steps into k_sem_take.

Another thread calls mpsc_pbuf_claim to consume data. In this condition,
mpsc_pbuf_claim has only small remaining space and needs to call rd_idx_inc
to reserve space, but there is still no data available.

The consumer should call k_sem_give to wake mpsc_pbuf_alloc again,
so the producer can allocate space and continue producing data.

Without this wake-up, the producer thread may wait forever in
k_sem_take, leading to a deadlock situation.

Signed-off-by: Fei Wang <fei.wang@jaguarmicro.com>