qtwebkit: fixing build / reducing build log size

GCC 8 introduced a new type of warning `-Wclass-memaccess` which
is included in `-Wall`. This warnings spits out *a million* of warnings
like the following:

```
[...]
/build/source/Source/WTF/wtf/Vector.h:128:15: warning: 'void* memcpy(void*, const void*, size_t)' writing to an object of type 'class WTF::RefPtr<WebCore::TransformOperation>' with no trivial copy-assignment; use copy-assignment or copy-initialization instead [-Wclass-memaccess]
         memcpy(dst, src, reinterpret_cast<const char*>(srcEnd) - reinterpret_cast<const char*>(src));
[...]
``

Logs demonstrating the issue:
https://hydra.nixos.org/build/100205478/nixlog/1

While I don't think disabling warnings is the best way to deal with this,
there is alrady precedent for this package and I don't feel confident enough
to either patch or bump this package.

Please view this as a low-friction sub-optimal suggestion in case nobody else
has a better fix.