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/wallet.cpp

@@ -34,6 +34,7 @@
 #include <util/moneystr.h>
 #include <util/rbf.h>
 #include <util/string.h>
+#include <util/system.h>
 #include <util/translation.h>
 #include <wallet/coincontrol.h>
 #include <wallet/context.h>