refactor: remove <util/system.h> include from wallet.h

Since we no longer store a ref to the global `ArgsManager`
inside the wallet, we can move the util/system.h
include to the cpp.

This dependency removal opened a can of worms, as few
other places were, invalidly, depending on the wallet's
header including it.

src/wallet/rpc/util.cpp

@@ -6,6 +6,7 @@
 
 #include <common/url.h>
 #include <rpc/util.h>
+#include <util/system.h>
 #include <util/translation.h>
 #include <wallet/context.h>
 #include <wallet/wallet.h>