[LyX/master] Avoid warning.

[Richard Kimberly Heck]

commit 97021ee3691a33f594e8b05258d6525abfb1f378
Author: Richard Kimberly Heck <rikih...@lyx.org>
Date:   Sat Apr 21 21:41:47 2018 -0400

    Avoid warning.
---
 src/frontends/qt4/GuiView.cpp |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/src/frontends/qt4/GuiView.cpp b/src/frontends/qt4/GuiView.cpp
index c1e2886..f18bbfe 100644
--- a/src/frontends/qt4/GuiView.cpp
+++ b/src/frontends/qt4/GuiView.cpp
@@ -3645,6 +3645,7 @@ bool GuiView::GuiViewPrivate::asyncBufferProcessing(
                                || status == Buffer::PreviewSuccess);
        }
 #else
+       (void) allow_async;
        Buffer::ExportStatus status;
        if (syncFunc) {
                status = (used_buffer->*syncFunc)(format, true);