meson: remove unknown arguments that cause fail to build
The build was failing with: retro-gtk/meson.build:188:4: ERROR: Got unknown keyword arguments "export_packages", "namespace"
Checking https://mesonbuild.com/Pkgconfig-module.html the arguments "export_packages" and "namespace" are not present in pkg.generate, so I removed them to make the build complete
The FTBFS was spotted on debian and this patch was already used to solves it