Qt5: decrease memory amount needed for QtQWebkit

On 64-bit platforms Qt's JavaScript engine tries to reserve 1GiB of
virtual memory via 'mmap()', to be backed by physical memory on demand.
Genode's 'mmap()' implementation currently does not support on-demand
allocation of physical memory and tries to allocate the whole amount at
once, which is usually far more than needed.

With this patch, the amount to be reserved gets decreased to 32MiB.

Fixes #1041.