permissions: Work around template partial specialization GCC bug

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71954

Pick-to: 6.5
Change-Id: I029f881fc44494a94c3e0233af3f3587b63ce7c4
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/kernel/qpermissions.h

@@ -36,9 +36,6 @@ class QPermission
     template <typename T, typename Enable = void>
     static constexpr inline bool is_permission_v = false;
 
-    template <typename T>
-    static constexpr inline bool is_permission_v<T, typename T::QtPermissionHelper> = true;
-
     template <typename T>
     using if_permission = std::enable_if_t<is_permission_v<T>, bool>;
 public:
@@ -72,6 +69,9 @@ private:
     friend class QCoreApplication;
 };
 
+template <typename T>
+constexpr bool QPermission::is_permission_v<T, typename T::QtPermissionHelper> = true;
+
 #define QT_PERMISSION(ClassName) \
     using QtPermissionHelper = void; \
     friend class QPermission; \