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.