Fix the following build error on mac os:
worker/imap/worker.go:368:29: undefined: syscall.TCP_KEEPCNT
worker/imap/worker.go:376:29: undefined: syscall.TCP_KEEPINTVL
These symbols are not defined on darwin.
Fixes: 5dfeff75f3 ("imap: add tcp connection options")
Signed-off-by: Robin Jarry <robin@jarry.cc>