perf evsel: Remove needless stddef.h from util/evsel.h

perf evsel: Remove needless stddef.h from util/evsel.h

We added it in 6db31874b25a ("perf evsel: Introduce is_group_member
method") but we already ditched that function, and there was nothing
else left that needed NULL nor anything else from stddef.h, ditch it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-1zy0xfsy61x81f3fpyx5znco@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>