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: https://github.com/SerenityOS/serenity/pull/26465
2: https://github.com/php/php-src/pull/18961
c24d51e59c