Add a comment at the top explaining where the generated file comes from

It's useful to have the source of the generated file on top when
reaching it from the IDE. Also it is useful to explain that it's a
generated file at all.

[ChangeLog][qtwaylandscanner] Include a comment on top explaining what
xml file is being represented.

Change-Id: Ie0584295791f246b0e8c83e0afd3a521f6bdbb2e
Reviewed-by: Johan Helsing <johan.helsing@qt.io>

src/tools/qtwaylandscanner/qtwaylandscanner.cpp

@@ -467,6 +467,9 @@ bool Scanner::process()
     if (m_xml->hasError())
         return false;
 
+    printf("// This file was generated by qtwaylandscanner\n");
+    printf("// source file is %s\n\n", qPrintable(m_protocolFilePath));
+
     for (auto b : qAsConst(m_includes))
         printf("#include %s\n", b.constData());