package/freerdp: avoid development warning

FreeRDP still uses old-style variables (about linking interfaces), and
that causes a warning, which explicitly states it is targeted at
developers:

    Policy CMP0022 is not set: INTERFACE_LINK_LIBRARIES defines the link
    interface.  Run "cmake --help-policy CMP0022" for policy details.  Use the
    cmake_policy command to set the policy and suppress this warning.

    Target "freerdp-client" has an INTERFACE_LINK_LIBRARIES property which
    differs from its LINK_INTERFACE_LIBRARIES properties.

    INTERFACE_LINK_LIBRARIES:

      [elided list of stuff]

    LINK_INTERFACE_LIBRARIES:

    This warning is for project developers.  Use -Wno-dev to suppress it.

So, just get rid of it as instructed in that warning message itself.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>