GUACAMOLE-1196: Fix compile issues with older VNC client versions where screen isn't defined.

[corentin-soriano]

Fix for compilation error related to #544:

In file included from display.h:25,
                 from display.c:23:
display.c: In function ‘guac_vnc_send_desktop_size’:
display.c:206:39: error: ‘rfbClient’ {aka ‘struct _rfbClient’} has no member named ‘screen’
  206 |             rfbClientSwap16IfLE(client->screen.width),
      |                                       ^~
display.c:206:39: error: ‘rfbClient’ {aka ‘struct _rfbClient’} has no member named ‘screen’
  206 |             rfbClientSwap16IfLE(client->screen.width),
      |                                       ^~
display.c:206:39: error: ‘rfbClient’ {aka ‘struct _rfbClient’} has no member named ‘screen’
  206 |             rfbClientSwap16IfLE(client->screen.width),
      |                                       ^~
display.c:207:39: error: ‘rfbClient’ {aka ‘struct _rfbClient’} has no member named ‘screen’
  207 |             rfbClientSwap16IfLE(client->screen.height),
      |                                       ^~
display.c:207:39: error: ‘rfbClient’ {aka ‘struct _rfbClient’} has no member named ‘screen’
  207 |             rfbClientSwap16IfLE(client->screen.height),
      |                                       ^~
display.c:207:39: error: ‘rfbClient’ {aka ‘struct _rfbClient’} has no member named ‘screen’
  207 |             rfbClientSwap16IfLE(client->screen.height),
      |                                       ^~
make[2]: *** [Makefile:618: libguac_client_vnc_la-display.lo] Error 1