clang 15 now warns:

  46    65.20 fedora:rawhide                : FAIL clang version 15.0.0 (Fedora 15.0.0-3.fc38)
    util/parse-events-bison.c:1401:9: error: variable 'parse_events_nerrs' set but not used [-Werror,-Wunused-but-set-variable]
        int yynerrs = 0;
            ^
    #define yynerrs         parse_events_nerrs
                            ^
    1 error generated.
    make[3]: *** [/git/perf-6.0.0-rc7/tools/build/Makefile.build:139: util] Error 2

Just ignore one more compiler warning for the bison generated C code.

Committer notes:

Older clangs don't know about -Wunused-but-set-variable, so we need to
add -Wno-unknown-warning-option to avoid this:

  37    44.92 fedora:32                     : FAIL clang version 10.0.1 (Fedora 10.0.1-3.fc32)
    error: unknown warning option '-Wno-unused-but-set-variable'; did you mean '-Wno-unused-const-variable'? [-Werror,-Wunknown-warning-option]
    make[3]: *** [/git/perf-6.0.0-rc7/tools/build/Makefile.build:139: util] Error 2
Reported-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@kernel.org>
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: https://lore.kernel.org/lkml/20220929140514.226807-1-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>