sadc: Don't count items several times

This patch makes sure that sadc doesn't count items several times.

Items here means CPU, network interfaces, etc.
sadc needs to count these items to allocate its structures for each
selected activity. But there are different activities based on the same
items (eg. A_CPU, A_PWR_CPUFREQ and A_PWR_WGHFREQ are all related to CPU
activities, or A_NET_DEV and A_NET_EDEV are related to network
interfaces). And so, when different activities based on the same
items were selected, the number of these items was counted several
times, which is inefficient.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>