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.

Meta/Lagom/Tools/CodeGenerators/IPCCompiler/main.cpp

@@ -16,6 +16,7 @@
 #include <ctype.h>
 #include <stdio.h>
 
+namespace {
 struct Parameter {
     Vector<ByteString> attributes;
     ByteString type;
@@ -823,6 +824,7 @@ void build(StringBuilder& builder, Vector<Endpoint> const& endpoints)
     for (auto const& endpoint : endpoints)
         build_endpoint(generator.fork(), endpoint);
 }
+} // end anonymous namespace
 
 ErrorOr<int> serenity_main(Main::Arguments arguments)
 {