Silence GCC 8 warnings in QString

qtbase/src/corelib/tools/qstring.cpp:3539:67: error: ‘void* memcpy(void*, const void*, size_t)’ copying an object of non-trivial type ‘class QChar’ from an array of ‘short unsigned int’ [-Werror=class-memaccess]
             memcpy(uc, d->data() + copystart, size * sizeof(QChar));

Change-Id: Ic601bed1a1f9e1b6f0ac1f9e58f1dcadb50ad724
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>