Klipper: Do not compile sources twice to build the data engine
libklipper_common_SRCS are compiled into the libklipper_common_static library and then linked for the Klipper application. The same sources are also compiled again to build plasma_engine_clipboard via plasma_engine_clipboard_SRCS. This results in almost double the compile time and repeated diagnostic messages (if there are any). Use the previously built static library for linking the data engine.