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

We added it in 07ac002f ("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.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evsel.h

@@ -4,7 +4,6 @@
 
 #include <linux/list.h>
 #include <stdbool.h>
-#include <stddef.h>
 #include <linux/perf_event.h>
 #include <linux/types.h>
 #include <internal/evsel.h>