[LyX/master] Compile on older gcc.

[Pavel Sanda]

commit 2db26ff5224288df83333e5149217ba947581348
Author: Pavel Sanda <sanda at lyx.org>
Date:   Mon Dec 14 00:01:05 2020 +0100

    Compile on older gcc.
---
 src/support/FileMonitor.h |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/src/support/FileMonitor.h b/src/support/FileMonitor.h
index 07a8dc5..622637e 100644
--- a/src/support/FileMonitor.h
+++ b/src/support/FileMonitor.h
@@ -20,6 +20,7 @@
 #include "support/signals.h"
 
 #include <memory>
+#include <map>
 
 #include <QFileSystemWatcher>
 #include <QObject>