libpsl is a lib that provides the Public Suffix List: the list
of all TLDs.
It's an optional feature of libcurl that we dont need for libkiwix.

Leaving it included causes issues when compiling for apple_all_static
on x86_64 as libpsf is installed (and thus found by libcurl's configure)
but is obviously only for x86_64.

An alternative could be to either cross-compile it first and properly
reference that version.

That sounds like too much effort for a feature we know we dont need.

https://github.com/rockdaboot/libpsl