use LPProtocol.new instead of LPProtocol()

Avoid potenial issue with vacp2p/nim-libp2p#1064 (comment)
in a future dependency bump.

Author: etan-status

beacon_chain/networking/eth2_network.nim

@@ -2013,7 +2013,8 @@ proc p2pProtocolBackendImpl*(p: P2PProtocol): Backend =
 
         try:
           mount `networkVar`.switch,
-                LPProtocol(codecs: @[`codecNameLit`], handler: snappyThunk)
+                LPProtocol.new(
+                  codecs = @[`codecNameLit`], handler = snappyThunk)
         except LPError as exc:
           # Failure here indicates that the mounting was done incorrectly which
           # would be a programming error