Kanshi (https://git.sr.ht/~emersion/kanshi) has added support for 
controlling the daemon remotely. To enable this, kanshi has to be build 
with libvarlink present. This patch adds libvarlink to the 
serialization.scm and extends the kanshi package inputs.

I tested the build on amd64 and on aarch64 with qemu as per the guide. 
Unfortunately my laptop ran out of disk space while compiling the 
toolchain for armhf so I did not test that.