ctdb-common: Return value of ctdb_queue_length() should be unsigned

ctdb-common: Return value of ctdb_queue_length() should be unsigned

Compiling with -Wsign-compare complains:

ctdb/server/ctdb_daemon.c: scope_hint: In function ‘daemon_queue_send’
ctdb/server/ctdb_daemon.c:259:40: warning: comparison of integer expressions of different signedness: ‘int’ and ‘uint32_t’ {aka ‘unsigned int’} [-Wsign-compare]
...

The struct ctdb_queue member out_queue_length is actually uint32_t, so
just return that type.

Found by csbuild.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>