socket.h: add missing sbuf.h inclusion

to get struct sbuf

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
diff --git a/src/socket.h b/src/socket.h
index ac9a57e..1a885a0 100644
--- a/src/socket.h
+++ b/src/socket.h
@@ -7,6 +7,7 @@
 #define SLIRP_SOCKET_H
 
 #include "misc.h"
+#include "sbuf.h"
 
 #define SO_EXPIRE 240000
 #define SO_EXPIREFAST 10000