Initialize m_exclusive in QGtk3MenuItem constructor

Problem detected while investigating a crash of a program using
QT, valgrind was used to detect errors and valgrind reported
"Conditional jump or move depends on uninitialised value(s)"
due to m_exclusive being uninitialized.

Not sure to what extent this fix matters in practice, the crash
turned out to be caused by something else outside QT. But anyway
it seems good to initialize m_exclusive like other things, in the
constructor, and it helps avoiding such warnings from valgrind,
that will make it easier to detect other problems using valgrind
or similar tools.

Amends ac8a1787eb74d374a346ae5982d0ea361747729e where it looks
like initialization of this member was simply forgotten.

Pick-to: 6.3 6.2 5.15
Change-Id: I3d1637a00b7899eded61c2e4f5f9a1c5041e4f45
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Elias Rudberg <mail@eliasrudberg.se>

src/plugins/platformthemes/gtk3/qgtk3menu.cpp

@@ -49,6 +49,7 @@ QGtk3MenuItem::QGtk3MenuItem()
       m_checkable(false),
       m_checked(false),
       m_enabled(true),
+      m_exclusive(false),
       m_underline(false),
       m_invalid(true),
       m_menu(nullptr),