BUILD: backend: fix build breakage in idle conn locking fix

The IDLE_CONNS_LOCK is called OTHER_LOCK in 2.3, but as it's only used
by developers when -DDEBUG_THREAD is used, it didn't show up when commit
5e22cf3a9 ("MEDIUM: backend: use a trylock to grab a connection on high
FD counts as well") was backported to 2.3.

There's no upstream commit since it's a pure 2.3 fix.
1 file changed