automoc: avoid more compiler warnings in linker-warning-silencing code

Revise the approach from commit 844244ccdc (automoc: avoid compiler
warnings in linker-warning-silencing code, 2022-08-10, v3.24.1~6^2)
to avoid `-Wmissing-declarations` warnings too.

Issue: #23823

Source/cmQtAutoMocUic.cxx

@@ -2183,6 +2183,9 @@ void cmQtAutoMocUicT::JobMocsCompilationT::Process()
     content += "// No files found that require moc or the moc files are "
                "included\n"
                "void " +
+      functionName +
+      "();\n"
+      "void " +
       functionName + "() {}\n";
   } else {
     // Valid content