devel/qt6-base: Prefer shared library for libzstd

Backport upstream commit 3f45905953d57e0174059d7d9d6bc75c3c1c406c as
without this patch QT by default pulls in (lib)zstd's static library
when CMake files are available which causes build failure unless (lib)zstd
is compiled with -fPIC (non standard option) and is inconsistent with
behaviour using pkgconfig

PR: 270664
Approved by: kde (tcberner)