Wrong: Bienvenu !
Right: Bienvenue !
Explanation: When wishing someone welcome with no verb, you always have to use the feminine: Bienvenue ! It's only when you use a complete sentence that you can say either Soyez le bienvenu or Soyez la bienvenue, according to the gender of the person you are talking to, or Soyez les bienvenus if it's more than one person.
Incidentally, I saw this mistake most recently in Utah, on a sign which surely deserves some kind of award for the most errors in the smallest amount of space. Bienvenu internationale voyageur's - just three words with enough mistakes to fail a mid-term exam!
In addition to bienvenu needing to end in e, in this instance it also needs to be followed by the preposition à. There's no reason for the adjective international to be feminine; in fact, it should be plural, to agree with voyageurs, and should follow rather than precede the noun. Finally, the apostrophe has absolutely no business there and makes me suspect that it was originally written in English as "Welcome international traveler's"* and then run through an online translator.
Better French greetings:
- Bienvenue aux voyageurs internationaux
Voyageurs internationaux : soyez les bienvenus
*It should have said "travelers" - learn about apostrophe s mistakes.