Fix clean parallel build
The newly introduced krfb-virtualmonitor also needs ui_connectionwidget.h (via rfbclient -> connectiondialog) and fails with "krfb/connectiondialog.h:25:10: fatal error: ui_connectionwidget.h: No such file or directory #include "ui_connectionwidget.h" when doing a clean build with a high enough number of jobs. Fix it by introducing a krfb_UI_SRCS variable, which is used by both executable targets.