moc: Restore compatibility with Qt 5.7's -b option

In Qt 5.7, it was possible to call moc "-bfoo.h" or "-b foo.h" and it had the
same effect. With the port to QCommandLineOption, we broke the -b option as it
was not annotated as a short option.
(Regression in a7e3c17e75)

Task-number: QTBUG-63706
Change-Id: I161d0f1a4e65d129063b5e8431802257677da19d
Reviewed-by: Jake Petroules <jake.petroules@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/tools/moc/main.cpp

@@ -267,6 +267,7 @@ int runMoc(int argc, char **argv)
     QCommandLineOption prependIncludeOption(QStringLiteral("b"));
     prependIncludeOption.setDescription(QStringLiteral("Prepend #include <file> (preserve default include)."));
     prependIncludeOption.setValueName(QStringLiteral("file"));
+    prependIncludeOption.setFlags(QCommandLineOption::ShortOptionStyle);
     parser.addOption(prependIncludeOption);
 
     QCommandLineOption includeOption(QStringLiteral("include"));