Fix debian bug 764142

When FreeRDP is compiled with -DSTATIC_CHANNELS=off and
RDP local sound is enable in Remmina, Remmina will crash
upon RDP connection with the error
/usr/lib/x86_64-linux-gnu/freerdp/rdpsnd-client-pulse.so: undefined symbol:
freerdp_dsp_context_new
This is the fix.