REORG: acitvity: uninline sched_activity_entry()

REORG: acitvity: uninline sched_activity_entry()

This one is expensive in code size because it comes with xxhash.h at a
low level of dependency that's inherited at plenty of places, and for
a function does doesn't benefit from inlining and could possibly even
benefit from not being inline given that it's large and called from the
scheduler.

Moving it to activity.c reduces the LoC by 1.2% and the binary size by
~1kB.