[LyX/master] Fix missing sstream include (gcc 17)

[José Matos]

commit 548e2b8615556a7552f767e973f330037054e856
Author: José Matos <jamatos at lyx.org>
Date:   Wed Jul 8 16:04:45 2026 +0100

    Fix missing sstream include (gcc 17)
    
    In gcc 17, currently in development, <string> no longer includes
    <sstream>.
---
 src/support/docstring.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/support/docstring.h b/src/support/docstring.h
index 0b96ffb0c9..a3db550e70 100644
--- a/src/support/docstring.h
+++ b/src/support/docstring.h
@@ -33,6 +33,7 @@ namespace lyx { typedef std::uint32_t char_type; }
 
 #endif
 
+#include <sstream>
 #include <string>
 
 namespace lyx {