qt: Allow enabling of QtWebKit with GCC 6+

qt: Allow enabling of QtWebKit with GCC 6+

Building Qt with QtWebKit on configuration step there is
a check which disables QtWebKit build with GCC 6+.
Back in the day nobody thought about building Qt with GCC
version greater than 5.x. And now with modern GCCs like
6.x and 7.x this assumption gets in the way.

Given in Buildroot today we don't have GCC older than 4.9
it should be safe to remove now meaningless check completely
by adding patch to qt.

Signed-off-by: Evgeniy Didin <didin@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>