qt: Fix QFileDialog for static builds

Summary:

This change partially reverts 248e22bbc0d7bc40ae3584d53a18507c46b0e553.

Backport of core PR19536.

Test Plan:

cmake -GNinja .. \
  -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Linux64.cmake
ninja

With no existing data directory:

./src/qt/bitcoin-qt

In the Welcome dialog, select "Use a custom data directory" and browse
for a path. Before this patch browsing was broken.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7754