webkit: silence gazillions of warnings

Building webkit with a recent gcc versions gives gazillions of
warnings such as 'warning: cast from 'WTF::AlignedBufferChar* {aka
char*}' to 'JSC::Identifier*' increases required alignment of target
type [-Wcast-align]'.

Those make the webkit build very noise, and hard to debug, so let's
silence those warnings.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>