stats_test: fix mismatching osmo_stats_reporter->send_item signature

The function pointer expects the last arg as int64_t, stats_test.c uses
an int instead. Fix the argument type as well as the printf format for it.

Fixes this compiler warning seen on our FreeBSD build slave:

    CC       stats/stats_test.o
  ../../tests/stats/stats_test.c:288:18: warning: incompatible pointer types assigning to 'int (*)(struct osmo_stats_reporter *, const struct osmo_stat_item_group *, const struct osmo_stat_item_desc *, int64_t)' from 'int (struct osmo_stats_reporter *, const struct osmo_stat_item_group *, const struct osmo_stat_item_desc *, int)' [-Wincompatible-pointer-types]
          srep->send_item = stats_reporter_test_send_item;
                          ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  1 warning generated.

Change-Id: I91cbfd4dd25a881b803943430101dabf07dafc7c
1 file changed