CLEANUP: qpack: fix format string in debugging code (int signedness)

CLEANUP: qpack: fix format string in debugging code (int signedness)

In issue #1939, Ilya mentions that cppchecks warned about use of "%d" to
report the QPACK table's index that's locally stored as an unsigned int.
While technically valid, this will never cause any trouble since indexes
are always small positive values, but better use %u anyway to silence
this warning.