QSlotObjectBase: fix return value of ref()

QAtomicInt::ref() returns bool, not int, so do the same in our ref().

As a drive-by, drop the superfluous inline keyword.

Pick-to: 6.6 6.5
Change-Id: I60712df3640b67dfd857355d364e0fc5a3a40650
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qobjectdefs_impl.h

@@ -424,7 +424,7 @@ namespace QtPrivate {
             static void cleanup(QSlotObjectBase *p) noexcept { Deleter{}(p); }
         };
 
-        inline int ref() noexcept { return m_ref.ref(); }
+        bool ref() noexcept { return m_ref.ref(); }
 #if QT_VERSION < QT_VERSION_CHECK(7, 0, 0)
         inline void destroyIfLastRef() noexcept
         { if (!m_ref.deref()) m_impl(Destroy, this, nullptr, nullptr, nullptr); }