SDL_dbus.h: define DBUS_TIMEOUT_USE_DEFAULT if not already defined.

(cherry picked from commit c521e1286b)

src/core/linux/SDL_dbus.h

@@ -29,6 +29,10 @@
 #include "SDL_stdinc.h"
 #include <dbus/dbus.h>
 
+#ifndef DBUS_TIMEOUT_USE_DEFAULT
+#define DBUS_TIMEOUT_USE_DEFAULT -1
+#endif
+
 typedef struct SDL_DBusContext
 {
     DBusConnection *session_conn;