perf mem-events: Remove unnecessary 'struct mem_info' forward declaration

perf mem-events: Remove unnecessary 'struct mem_info' forward declaration

'struct mem_info' is defined at 22nd line.

The declaration here is unnecessary. Remove it.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: kael_w@yeah.net
Link: http://lore.kernel.org/lkml/20210406105104.675879-1-wanjiabing@vivo.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>