libperf test evsel: Fix build error on !x86 architectures

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