Qt: Try to fix a Visual C++ encoding warning.

Try to fix

.\simple_dialog.cpp(49) : warning C4566: character represented by universal-character-name '\uFFFF' cannot be represented in the current code page (1252)

We're assigning a delimiter. "Cannot be represented" is a feature,
not a bug.

Change-Id: Iba8bc5f5b42a43ec971d822b0e00f932787c7b16
Reviewed-on: https://code.wireshark.org/review/4409
Reviewed-by: Gerald Combs <gerald@wireshark.org>