perf stat: Merge cases in process_evlist

As two cases in process_evlist has same behavior, make the first fall
through to the second.

Commiter notes:

Added __fallthrough, the kernel has "fallthrough", we need to make
tools/ use it.
Signed-off-by: Shang XiaoJing <shangxiaojing@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220922141438.22487-3-shangxiaojing@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-stat.c

@@ -662,9 +662,7 @@ static void process_evlist(struct evlist *evlist, unsigned int interval)
 	if (evlist__ctlfd_process(evlist, &cmd) > 0) {
 		switch (cmd) {
 		case EVLIST_CTL_CMD_ENABLE:
-			if (interval)
-				process_interval();
-			break;
+			__fallthrough;
 		case EVLIST_CTL_CMD_DISABLE:
 			if (interval)
 				process_interval();