perf tests mmap-basic: Remove unused variable to address clang 15 warning

A clang 15 build reveal several unused-but-set variables, removing the
'foo' variable in tests/mmap-basic.o object to address one of those
cases.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20220929140514.226807-2-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Author: olsajiri

tools/perf/tests/mmap-basic.c

@@ -114,8 +114,7 @@ static int test__basic_mmap(struct test_suite *test __maybe_unused, int subtest
 
 	for (i = 0; i < nsyscalls; ++i)
 		for (j = 0; j < expected_nr_events[i]; ++j) {
-			int foo = syscalls[i]();
-			++foo;
+			syscalls[i]();
 		}
 
 	md = &evlist->mmap[0];