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: 9dfcb759 ("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.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/lib/include/internal/evsel.h

@@ -8,6 +8,7 @@
 
 struct perf_cpu_map;
 struct perf_thread_map;
+struct xyarray;
 
 struct perf_evsel {
 	struct list_head	 node;