package/freerdp: needs C++

FreeRDP does not have a single C++ source file, however it insists on
having a C++ compiler. Removing that requirement is a bit too much
involving, so we just depend on it...

Fixes:
    http://autobuild.buildroot.org/results/e5b/e5bee760d52e5ee433dcb6c8a7296f99312cfcf4/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>