[LyX/master] Add missing header include

[Juergen Spitzmueller]

commit f47ea0dd5e851ae2a315faa46c03d579948331c9
Author: Juergen Spitzmueller <spitz at lyx.org>
Date:   Sun Dec 20 12:42:04 2020 +0100

    Add missing header include
---
 src/Server.h |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/src/Server.h b/src/Server.h
index 2f26b8a..215b7e9 100644
--- a/src/Server.h
+++ b/src/Server.h
@@ -15,6 +15,7 @@
 #define SERVER_H
 
 #include <memory>
+#include <string>
 #include <vector>
 
 #ifdef _WIN32