Include user.h in the new winstation.c to avoid a warning from
-Wmissing-declarations.

diff --git a/server/winstation.c b/server/winstation.c
index f25e210..57b5d45 100644
--- a/server/winstation.c
+++ b/server/winstation.c
@@ -32,6 +32,7 @@
 #include "handle.h"
 #include "request.h"
 #include "process.h"
+#include "user.h"
 #include "wine/unicode.h"
 
 struct winstation