libperf test evsel: Fix build error on !x86 architectures

In test_stat_user_read, following build error occurs except i386 and
x86_64 architectures:

tests/test-evsel.c:129:31: error: variable 'pc' set but not used [-Werror=unused-but-set-variable]
  struct perf_event_mmap_page *pc;

Fix build error.
Signed-off-by: Shunsuke Nakamura <nakamura.shun@fujitsu.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20211006095703.477826-1-nakamura.shun@fujitsu.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/perf/tests/test-evsel.c

@@ -148,6 +148,7 @@ static int test_stat_user_read(int event)
 	__T("failed to mmap evsel", err == 0);
 
 	pc = perf_evsel__mmap_base(evsel, 0, 0);
+	__T("failed to get mmapped address", pc);
 
 #if defined(__i386__) || defined(__x86_64__)
 	__T("userspace counter access not supported", pc->cap_user_rdpmc);