FTL_lua: Properly guard readline

Readline support should only be used when we know it is available. Lets
properly use a ifdef guard like we do in `shell.c`.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>

src/lua/ftl_lua.c

@@ -20,7 +20,9 @@
 #include "../files.h"
 // get_web_theme_str
 #include "../datastructure.h"
+#if HAVE_READLINE
 #include <readline/history.h>
+#endif
 #include <wordexp.h>
 #include "scripts/scripts.h"