backend-rdp: fix undefined functions linking errors
This addresses issue #262 (closed) (backend-rdp undefined function errors)
(1/2): Add WinPR as a dependency to the RDP backend. This library is build from FreeRDP repository and used by xfreerdp, but it is packaged as a seperate library. The RDP had a implicit dependency on it. People currently building FreeRDP with the RDP backend will have to install this new dependency.
(2/2): Remove b_lundef backend specific override.