`proc-hidepid.service`: Fixing `pkexec` related issues

[adrelanos]

After applying the changes in #208, does pkexec work for you? @wryMitts

---

Test command (X11 only compatible, not working in Wayland):

pkexec mousepad /tmp/testfile

I.e. try running any application with root rights through use of pkexec.

(Maybe Wayland compatible.)

pkexec nano /tmp/testfile

---

Test command (Wayland compatible, hopefully):

sudo apt update
sudo apt install gparted
gparted

---

related:
https://www.kicksecure.com/wiki/Security-misc#hidepid