QtCore/Windows: Fix inconsistent linkage of helper qAppFileName()

MSVC is complaining about inconsistent linkage in CMake Unity (Jumbo)
builds since the export is not declared.

Since it is neither used nor declared in any header, remove the export.

Pick-to: 6.5
Task-number: QTBUG-109394
Change-Id: Ie6dbad0188b7fa42c0abd40c48cb50129badb38e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qcoreapplication_win.cpp

@@ -21,7 +21,7 @@ QT_BEGIN_NAMESPACE
 
 using namespace Qt::StringLiterals;
 
-Q_CORE_EXPORT QString qAppFileName()                // get application file name
+QString qAppFileName()                // get application file name
 {
     /*
       GetModuleFileName() returns the length of the module name, when it has