perf evsel: Remove duplicate 'struct target' forward declaration

'struct target' is declared twice. One has been declared at 21st line.
Remove the duplicate.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: kael_w@yeah.net
Link: http://lore.kernel.org/lkml/20210401062424.991737-1-wanjiabing@vivo.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>