Fix "C99 inline functions are not supported" warning on OpenBSD

Fix "C99 inline functions are not supported" warning on OpenBSD

OpenBSD has below compiler warning due to hist_sum() being inlined.
It should be fine with no inline (or move it to iolog.h with static
if we somehow want to keep it inlined).

iolog.c:702: warning: C99 inline functions are not supported; using GNU89
iolog.c:702: warning: to disable this warning use -fgnu89-inline or the gnu_inline function attribute

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>