Everywhere: Warn on function definitions without prototypes

If no header includes the prototype of a function, then it cannot be
used from outside the translation unit it was defined in. In that case,
it should be marked as `static`, in order to avoid possible ODR
problems, unnecessary exported symbols, and allow the compiler to better
optimize those.

If this warning triggers in a function defined in a header, `inline`
needs to be added, otherwise if the header is included in more than one
TU, it will fail to link with a duplicate definition error.

The reason this diff got so big is that Lagom-only code wasn't built
with this flag even in Serenity times.

Ladybird/Qt/EventLoopImplementationQt.cpp

@@ -94,7 +94,7 @@ struct SignalHandlersInfo {
 };
 
 static Singleton<SignalHandlersInfo> s_signals;
-SignalHandlersInfo* signals_info()
+static SignalHandlersInfo* signals_info()
 {
     return s_signals.ptr();
 }