multi: add golang 1.17 compatible build tags

With go 1.17 a change to the build flags was implemented:
https://go.googlesource.com/proposal/+/master/design/draft-gobuild.md

The formatter now automatically adds the forward-compatible build tag
format and the linter checks for them, so we need to include them in our
code.

cmd/lncli/autopilotrpc_active.go

@@ -1,3 +1,4 @@
+//go:build autopilotrpc
 // +build autopilotrpc
 
 package main

cmd/lncli/autopilotrpc_default.go

@@ -1,3 +1,4 @@
+//go:build !autopilotrpc
 // +build !autopilotrpc
 
 package main

cmd/lncli/invoicesrpc_active.go

@@ -1,3 +1,4 @@
+//go:build invoicesrpc
 // +build invoicesrpc
 
 package main

cmd/lncli/invoicesrpc_default.go

@@ -1,3 +1,4 @@
+//go:build !invoicesrpc
 // +build !invoicesrpc
 
 package main

cmd/lncli/walletrpc_active.go

@@ -1,3 +1,4 @@
+//go:build walletrpc
 // +build walletrpc
 
 package main

cmd/lncli/walletrpc_default.go

@@ -1,3 +1,4 @@
+//go:build !walletrpc
 // +build !walletrpc
 
 package main

cmd/lncli/watchtower_active.go

@@ -1,3 +1,4 @@
+//go:build watchtowerrpc
 // +build watchtowerrpc
 
 package main

cmd/lncli/watchtower_default.go

@@ -1,3 +1,4 @@
+//go:build !watchtowerrpc
 // +build !watchtowerrpc
 
 package main