Autogen/RCC: Only pass --no-zstd to Qt6

In commit 9e87df4d46 (Autogen/RCC: Disable zstd if it is not supported,
2024-02-08, v3.29.0-rc1~5^2) we added this flag, but it doesn't exist
for Qt 5.  It was only introduced with qtbase commit `14546d1816a8`,
which first appeared in Qt 6.

Fixes: #27111
Issue: #25664