Replace has_optional_keyword for old libprotoc versions

Use the old syntax-based check if libprotoc version is less than
3.12.

Amends dd04edf72f

Pick-to: 6.6
Task-number: QTBUG-115693
Change-Id: Ia411f70337a14843ff320b7c75ab140f5021f2f6
Reviewed-by: Tatiana Borisova <tatiana.borisova@qt.io>

src/tools/qtprotobufgen/qprotobufgenerator.cpp

@@ -12,6 +12,8 @@
 #include "utils.h"
 #include "options.h"
 
+#include "qtprotocdefs.h"
+
 #include <array>
 #include <cassert>
 #include <numeric>
@@ -286,7 +288,12 @@ void QProtobufGenerator::GenerateHeader(const FileDescriptor *file,
                                                 + "/" + field->message_type()->name());
                         qtTypesSet.insert(field->message_type()->file()->package());
                     }
+#ifdef HAVE_PROTOBUF_SYNC_PIPER
                     if (field->has_optional_keyword())
+#else
+                    if (file->syntax() == FileDescriptor::SYNTAX_PROTO2 && field->is_optional()
+                        && !field->containing_oneof())
+#endif
                         hasOptionalFields = true;
                 }
             });