Fix linking with KCUPSLIB_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.

CMakeLists.txt gets a new generate_export_header(kcupslib) command
so that KCUPSLIB_EXPORT can be used in the first place.