t-reftable-pq: make merged_iter_pqueue_check() static

t-reftable-pq: make merged_iter_pqueue_check() static

merged_iter_pqueue_check() is a function previously defined in
reftable/pq_test.c (now t/unit-tests/t-reftable-pq.c) and used in
the testing of a priority queue as defined by reftable/pq.{c, h}.
As such, this function is only called by reftable/pq_test.c and it
makes little sense to expose it to non-testing code via reftable/pq.h.

Hence, make this function static and remove its prototype from
reftable/pq.h.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>