Avoid duplicate modules being added to HOOKS in mkinitcpio.conf

Updates the sed regex to avoid adding multiple modules to the HOOKS in /etc/mkinitcpio.conf

Fixes #255 (closed)

NOTE: This may be treating the symptom more than the cause. I'm not familiar enough with the code base to understand why the sed statements addressed in this patch are needed since the same modules are already added here util-base.sh#L368-370 and here util-desktop.sh#L435-437. If there is a better fix please feel free to close this MR.