libperf: Add missing 'struct xyarray' forward declaration

libperf: Add missing 'struct xyarray' forward declaration

We were getting it by luck, from files included before internal/evsel.h
where it is being included.

Fixes: 9dfcb7599084 ("libperf: Move fd array from perf's evsel to lobperf's perf_evsel class")
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: https://lkml.kernel.org/n/tip-r8ukhxprpkflbd2k9vcc42v1@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>