[LyX/master] Fix compilation on older gccs.

[Pavel Sanda]

commit 2ab39fe1211935908198eff87d427b6437b3eb4a
Author: Pavel Sanda <sanda at lyx.org>
Date:   Mon Nov 30 12:21:56 2020 +0100

    Fix compilation on older gccs.
---
 src/Format.h |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/src/Format.h b/src/Format.h
index b4cc9f4..99cc120 100644
--- a/src/Format.h
+++ b/src/Format.h
@@ -16,6 +16,7 @@
 #include "support/trivstring.h"
 
 #include <vector>
+#include <string>
 
 namespace lyx {