libperf: Fix build error with LIBPFM4=1

Fix build error with LIBPFM4=1:

    CC      util/pfm.o
  util/pfm.c: In function ‘parse_libpfm_events_option’:
  util/pfm.c:102:30: error: ‘struct evsel’ has no member named ‘leader’
    102 |                         evsel->leader = grp_leader;
        |                              ^~

Committer notes:

There is this entry in 'make -C tools/perf build-test' to test the build
with libpfm:

  $ grep libpfm tools/perf/tests/make
  make_with_libpfm4   := LIBPFM4=1
  run += make_with_libpfm4
  $

But the test machine lacked libpfm-devel, now its installed and further
cases like this shouldn't happen.

Committer testing:

Before this patch this fails, after applying it:

  $ make -C tools/perf build-test
  make: Entering directory '/var/home/acme/git/perf/tools/perf'
  - tarpkg: ./tests/perf-targz-src-pkg .
                   make_static: make LDFLAGS=-static NO_PERF_READ_VDSO32=1 NO_PERF_READ_VDSOX32=1 NO_JVMTI=1 -j24  DESTDIR=/tmp/tmp.KzFSfvGRQa
  <SNIP>
             make_no_scripts_O: make NO_LIBPYTHON=1 NO_LIBPERL=1
           make_with_libpfm4_O: make LIBPFM4=1
         make_install_prefix_O: make install prefix=/tmp/krava
            make_no_auxtrace_O: make NO_AUXTRACE=1
  <SNIP>
  $ rpm -q libpfm-devel
  libpfm-devel-4.11.0-4.fc34.x86_64
  $

FIXME:

This shows a need for 'build-test' to bail out when a build option is
specified that has no required library devel files installed.

Fixes: fba7c866

 ("libperf: Move 'leader' from tools/perf to perf_evsel::leader")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20210713091907.1555560-1-hca@linux.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>