Add build depend on "libcpprest-dev" (Casablanca dev package).

[FransUrbo]

Avoids the following build problem:

CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
CASABLANCA_INCLUDE_DIR (ADVANCED)
   used as include directory in directory /usr/local/src/FreeRDP-WebConnect/wsgate
   used as include directory in directory /usr/local/src/FreeRDP-WebConnect/wsgate
   used as include directory in directory /usr/local/src/FreeRDP-WebConnect/wsgate
   used as include directory in directory /usr/local/src/FreeRDP-WebConnect/wsgate
LIB_CASABLANCA (ADVANCED)
    linked by target "wsgate" in directory /usr/local/src/FreeRDP-WebConnect/wsgate