vnc: remove a memory leak in zlib

Makes sure we free all ressources used in zlib encoding (zlib stream
and buffer).

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/vnc.h b/vnc.h
index c97113b..2fbe614 100644
--- a/vnc.h
+++ b/vnc.h
@@ -405,5 +405,6 @@
 void vnc_hextile_set_pixel_conversion(VncState *vs, int generic);
 
 void vnc_zlib_send_framebuffer_update(VncState *vs, int x, int y, int w, int h);
+void vnc_zlib_clear(VncState *vs);
 
 #endif /* __QEMU_VNC_H */