As far as I can tell that's been there since the initial open source
release (commit 528006a). The header defines ftok() and a handful of
IPC_* constants, none of which are used here.

System V IPC is increasingly being replaced by POSIX IPC and may
therefore not be implemented on new and/or hobbyist operating systems
such as SerenityOS[1]. In the past that wasn't an issue as the OPCache
could be disabled, which is no longer possible as of PHP 8.5[2].

I was able to build with the include patched out, but we would prefer
this to be addressed upstream.

1: SerenityOS/serenity#26465
2: #18961