perf_counter: Add dummy perf_counter_print_debug function

Impact: minimize requirements on architectures

Currently, an architecture just enabling CONFIG_PERF_COUNTERS but not
providing any extra functions will fail to build with
perf_counter_print_debug being undefined, since we don't provide an
empty dummy definition like we do with the hw_perf_* functions.

This provides an empty dummy perf_counter_print_debug() to make it
easier for architectures to turn on CONFIG_PERF_COUNTERS.
Signed-off-by: Paul Mackerras <paulus@samba.org>

kernel/perf_counter.c

@@ -54,6 +54,8 @@ int __weak hw_perf_group_sched_in(struct perf_counter *group_leader,
 	return 0;
 }
 
+void __weak perf_counter_print_debug(void)	{ }
+
 static void
 list_add_counter(struct perf_counter *counter, struct perf_counter_context *ctx)
 {