walletrpc: add missing REST body annotation

The missing body annotation caused the library to ignore any parameters
sent to this RPC endpoint when using the REST proxy.

lnrpc/walletrpc/walletkit.yaml

@@ -5,6 +5,7 @@ http:
   rules:
     - selector: walletrpc.WalletKit.ListUnspent
       post: "/v2/wallet/utxos"
+      body: "*"
     - selector: walletrpc.WalletKit.LeaseOutput
       post: "/v2/wallet/utxos/lease"
       body: "*"