Fix devel/libkolab, to properly export symbols in the library.

The update to libkolab in r421840 unfortunately broke desktuils/libkolab on the
current version of FreeBSD.

The issue is addressed in two upstream commits

• 62ebad910fa7d1843373056dd795fc5c36b20bb2 ==> Workaround for qt not defining Q_DECL_IMPORT
• d0a2fda6059abda610afe6a8d29b8f5a1c6ae4ae ==> Fix multiple-definition-warning due to redefining Q_DECL_IMPORT.

Instead of applying said two patches and enabling them on Q_OS_FREEBSD, rakcuo@
suggested to instead just define MAKE_KOLAB_LIB while the proper fix isn't
upstreamed.

Reported by: Steve Wills <swills@FreeBSD.org> via jenkins
Approved by: rakcuo (mentor)