build: update LibVNCServer submodule

This includeas a libvncclient WaitForMessage() fix to make it return
1 in case of buffered data, too.

Previously, WaitForMessage() would only check the socket for new data
using select(), even when ReadFromRFBServer had already buffered
unprocessed data. This could cause unnecessary timeouts and delays in
message processing.

Closes #282

Author: bk138