Update for changes in FreeRDP master's rfx_context_new()

FreeRDP's rfx_context_new now takes an argument, update Remmina's
rdp_plugin.c for this change.

Without this fix, Remmina fails to build against FreeRDP master with the following error:

remmina-plugins/rdp/rdp_plugin.c: In function 'remmina_rdp_pre_connect':
remmina-plugins/rdp/rdp_plugin.c:239:3: error: too few arguments to function 'rfx_context_new'