Fix linking with KCM_FIREWALL_CORE_EXPORT instead of Q_DECL_EXPORT

The issue with Q_DECL_EXPORT is that it doesn't automatically switch to an import declaration when being used by external code. This might not matter on every system but it fails on mine.

Note: There are two more Q_DECL_EXPORT in the codebase, used for subclasses in the firewalld and ufw backend directories next to core. Those directories don't use generate_export_header() in their CMakeLists.txt and the exported subclasses aren't explicitly included by any external program or library, so they don't cause any trouble for the build. I decided to keep these two Q_DECL_EXPORT occurrences untouched.