RDP: fix Q_WS_X11 is not working in Qt5

Q_WS_X11 is not working in Qt5, and xfreerdp is also supported on windows
so, replace Q_WS_X11 by FREERDP_EXECUTABLE_FOUND in CMakeList.txt
So that rdp could be built on both Qt4 and Qt5 environment.

Signed-off-by: AceLan Kao <acelan@acelan.idv.tw>