Add missing 'break' in switch.

Don't follow

 add_library(... UNKNOWN IMPORTED)

with

 add_library(... INTERFACE IMPORTED)

The Qt4 IMPORTED targets are of UNKNOWN type, so this
caused double-target-defintion errors.

 http://thread.gmane.org/gmane.comp.kde.devel.buildsystem/7968
Merge topic 'INTERFACE_LIBRARY-target-type' into next

ea5dcee Add missing 'break' in switch.
Make it possible to export() INTERFACE_LIBRARY targets.
Make it possible to install(EXPORT) INTERFACE_LIBRARY targets.
VS: Do not generate INTERFACE_LIBRARY targets at all

Do not generate project files for INTERFACE_LIBRARY targets.  Do not
generate dependencies on these targets; this is safe because transitive
dependencies on real targets have already been hooked up.
Merge topic 'INTERFACE_LIBRARY-target-type' into next

625f9de VS: Do not generate INTERFACE_LIBRARY targets at all
06b34fa Make it possible to install(EXPORT) INTERFACE_LIBRARY targets.
81813a0 Make it possible to export() INTERFACE_LIBRARY targets.