autologin's pam.d config doesn't source /etc/pam.d/limits.conf

I'm an indirect autologin user because I run Postmarketos on a phone. Postmarketos in turn uses Alpine's vanilla autologin in its tinydm package. I had a bit of a hard time to understand why I couldn't have realtime priorities in pmos, and at last I found out that I had to add a line to /etc/pam.d/autologin to make it work. I don't get into more details because I'll do a PR to solve this issue in just a moment. I'll be happy to discuss the issue if someone feels like it.