meson: Force '.so' extension for DRI drivers

DRI driver loadable modules are always installed with install_megadriver.py with names ending with '.so', irrespective of platform.

Force the name the loadable module is built with to match, so install_megadriver.py doesn't spin trying to remove non-existent symlinks.

Fixes: c77acc3c "meson: remove meson-created megadrivers symlinks"