BUILD: contrib/prometheus: fix build breakage caused by move of idle_pct

The idle_pct thread-local variable was moved to struct thread_info by
commit 81036f2 ("MINOR: time: move the cpu, mono, and idle time to
thread_info") but not updated in service-prometheus.c, thus breaking
it.

No backport is needed. This fixes GH issue #110.
1 file changed