QFileSystemModelPrivate: de-inline ctor and dtor

This class is dllexport'ed and has members that refer to classes that
are only forward-declared in the header (such as QRegularExpression,
used in std::vector). GCC and MSVC appear to emit the destructor at the
point where it is used, which I guess is the vtable, which probably
means where the constructor was inlined because I don't see any virtual
overrides. Clang (at least in the form of ICX) appears to emit it at the
class declaration point.

C:\Qt\qt-everywhere-src-6.3.0\qtbase\src\gui\itemmodels/qfilesystemmodel_p.h(94,20): note: in instantiation of member function 'std::vector<QRegularExpression>::~vector' requested here
class Q_GUI_EXPORT QFileSystemModelPrivate : public QAbstractItemModelPrivate
                   ^
C:\Qt\qt-everywhere-src-6.3.0\qtbase\src\gui\itemmodels/qfilesystemmodel_p.h(94,20): note: in implicit destructor for 'QFileSystemModelPrivate' first required here
C:\Qt\qt-everywhere-src-6.3.0\qtbase\src\gui\itemmodels/qfilesystemmodel_p.h(94,7): note: due to 'QFileSystemModelPrivate' being dllexported

Fix by defining both the default ctor and the dtor out-of-line.

Task-number: QTBUG-45582
Fixes: QTBUG-103605
Pick-to: 5.15 6.2 6.3
Change-Id: I77c8221eb2824c369feffffd16f08ee39004a825
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/gui/itemmodels/qfilesystemmodel.cpp

@@ -2036,6 +2036,10 @@ QStringList QFileSystemModelPrivate::unwatchPathsAt(const QModelIndex &index)
 }
 #endif // filesystemwatcher && Q_OS_WIN
 
+QFileSystemModelPrivate::QFileSystemModelPrivate() = default;
+
+QFileSystemModelPrivate::~QFileSystemModelPrivate() = default;
+
 /*!
     \internal
 */

src/gui/itemmodels/qfilesystemmodel_p.h

@@ -179,7 +179,8 @@ public:
         bool isVisible = false;
     };
 
-    QFileSystemModelPrivate() = default;
+    QFileSystemModelPrivate();
+    ~QFileSystemModelPrivate();
     void init();
     /*
       \internal