log+rpcperms: add custom RPCP logger to rpcperms package

The custom RPC middleware logic that we are going to add in the next
commits will need to log under their own sub logger so we add one with a
new subsystem name.

log.go

@@ -37,6 +37,7 @@ import (
 	"github.com/lightningnetwork/lnd/peernotifier"
 	"github.com/lightningnetwork/lnd/routing"
 	"github.com/lightningnetwork/lnd/routing/localchans"
+	"github.com/lightningnetwork/lnd/rpcperms"
 	"github.com/lightningnetwork/lnd/signal"
 	"github.com/lightningnetwork/lnd/sweep"
 	"github.com/lightningnetwork/lnd/watchtower"
@@ -159,6 +160,7 @@ func SetupLoggers(root *build.RotatingLogWriter, interceptor signal.Interceptor)
 	AddSubLogger(root, chanacceptor.Subsystem, interceptor, chanacceptor.UseLogger)
 	AddSubLogger(root, funding.Subsystem, interceptor, funding.UseLogger)
 	AddSubLogger(root, cluster.Subsystem, interceptor, cluster.UseLogger)
+	AddSubLogger(root, rpcperms.Subsystem, interceptor, rpcperms.UseLogger)
 }
 
 // AddSubLogger is a helper method to conveniently create and register the