powertop: call "xset" from $PATH

Patch the source code to find "xset" in $PATH rather than expecting the
hard-coded path "/usr/bin/xset" to work. I've decided *not* to hard-code
a proper path like "${xset}/bin/xset", because that reference greatly
increases the size of the powertop closure. Since "xset" is required
only for --calibrate (and that even seems to work fine without it), it
felt like an optional dependency is more appropriate in this case.

Thanks to @heydojo locating the source code file that needs patching.

Fixes https://github.com/NixOS/nixpkgs/issues/12662.