Fix: perf ust: check close return value

Fix coverity defect:
** CID 1208905:  Unchecked return value  (CHECKED_RETURN)
/liblttng-ust/lttng-context-perf-counters.c: 147 in setup_perf()

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>