So what's this all about? Well, the character of Sprite is a centuries old "eternal" who like all the eternals has helped shape human history. Sprite possesses powers of magic and illusion, so presumably she assisted Houdini at some point. But the reference is so prominent that I'm wondering if this is a setup for a future Eternals movie or even a Sprite spin-off series? So is Houdini about to join the MCU?
Eternals isn't the only new release to feature a glimpse of a Houdini poster. In Paramount's Clifford The Big Red Dog we see a "Do Spirits Return?" poster hanging in the apartment of an amateur magician played by comedian Russell Peters.
UPDATE: Here is the poster as it appears in Eternals. This image comes from Screen Rant who also tackled the question of Houdini's cameo.