Fix compilation with FreeRDP master

context_size has been renamed to ContextSize, this patch fixes it
diff --git a/src/compositor-rdp.c b/src/compositor-rdp.c
index 1a973d2..33ec77d 100644
--- a/src/compositor-rdp.c
+++ b/src/compositor-rdp.c
@@ -915,7 +915,7 @@
 	RdpPeerContext *peerCtx;
 	char seat_name[32];
 
-	client->context_size = sizeof(RdpPeerContext);
+	client->ContextSize = sizeof(RdpPeerContext);
 	client->ContextNew = (psPeerContextNew)rdp_peer_context_new;
 	client->ContextFree = (psPeerContextFree)rdp_peer_context_free;
 	freerdp_peer_context_new(client);