package/qt5/qt5base: fix build with gcc 11

Fix the following build failure with gcc 11:

/data/buildroot-autobuilder/instance-0/output-1/build/qt5base-5.15.2/include/QtCore/../../src/corelib/global/qfloat16.h:300:7: error: 'numeric_limits' is not a class template
  300 | class numeric_limits<QT_PREPEND_NAMESPACE(qfloat16)> : public numeric_limits<float>
      |       ^~~~~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/9a7a987af40b8408ccdfcae4890008c7090b41a1



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>