Compilation error on non ubuntu based distros

Upstream readline does not include stdio.h.

Only reason it works on Ubuntu/Debian is because they patch it in (btw, this is considered bad practice).
This will cause compilation errors on Linux distributions that stay closer to upstream.

The [manpage](https://man7.org/linux/man-pages/man3/readline.3.html) even states that you should include stdio.h first

Locate the readline include usage in the shell's C sources and compare it with the readline manpage linked in the issue. Verify the project compiles on a non-Ubuntu Linux distribution after ensuring stdio.h is included first.

Written by the indexing model from the issue text.