text/unicode: kill a "no previous prototype" warning

By adding include the same way as in charset.c

src/text/unicode.c

@@ -29,6 +29,8 @@
 #endif
 
 #include <vlc_common.h>
+
+#include "libvlc.h"
 #include <vlc_charset.h>
 
 #include <assert.h>