`hide-hardware-info.service`: hide `/proc/kallsyms`

[adrelanos]

Add /proc/kallsyms to the list as well, as this contains all the memory addresses for each kernel symbol.

Originally posted by @monsieuremre in #172

Maybe not needed if we implement:

• remount-secure: use procfs mount option subset (hide-hardware-info.service) #205

[monsieuremre]

Considering our default setup, I think this would not be necessary for the most part. We hide the sensitive content with a kernel parameter. I am not sure this adds more than what we already have.