perf jevents: Fix some would-be warnings

perf jevents: Fix some would-be warnings

Before enabling warnings through HOSTCFLAGS, fix the would-be warnings:

    HOSTCC  pmu-events/jevents.o
  pmu-events/jevents.c:74:22: warning: no previous prototype for ‘convert’ [-Wmissing-prototypes]
     74 | enum aggr_mode_class convert(const char *aggr_mode)
        |                      ^~~~~~~
  pmu-events/jevents.c: In function ‘print_events_table_entry’:
  pmu-events/jevents.c:373:8: warning: declaration of ‘topic’ shadows a global declaration [-Wshadow]
    373 |  char *topic = pd->topic;
        |        ^~~~~
  pmu-events/jevents.c:316:14: note: shadowed declaration is here
    316 | static char *topic;
        |              ^~~~~
  pmu-events/jevents.c: In function ‘json_events’:
  pmu-events/jevents.c:554:9: warning: declaration of ‘func’ shadows a global declaration [-Wshadow]
    554 |   int (*func)(void *data, struct json_event *je),
        |   ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  pmu-events/jevents.c:85:15: note: shadowed declaration is here
     85 | typedef int (*func)(void *data, struct json_event *je);
        |               ^~~~
  pmu-events/jevents.c: In function ‘main’:
  pmu-events/jevents.c:1211:25: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   1211 |  char *err_string_ext = "";
        |                         ^~
  pmu-events/jevents.c:1304:17: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   1304 |  err_string_ext = " for std arch event";
        |                 ^

Signed-off-by: John Garry <john.garry@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/1634807805-40093-2-git-send-email-john.garry@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>