The Autohypnosis skill was added by Ultimate Psionics, published by Dreamscarred Press*.
On this page it says:
Presented are new psionics-related skills and existing skills that are used in new ways by psionic characters.
So the skill is psionics-related but does that mean it cannot be used by non-psionic classes? I could easily think of fluff related reasons for why/how other types of characters would/could train their mind in such a way. It doesn't seem to require any sort of innate psionic power, like actual telekinesis or other psionic abilities. Just intensive training.
In other words, is it a house-ruling to allow other types of characters to rank in Autohypnosis?
As an allegory, there's nothing preventing a Fighter from having ranks in Spellcraft and/or Use Magic Device...
*: So therefore 3rd Party material and not published/supported by Paizo.