perf tools: Fix the build on the alpine:edge distro

The UAPI file byteorder/little_endian.h uses the __always_inline define
without including the header where it is defined, linux/stddef.h, this
ends up working in all the other distros because that file gets included
seemingly by luck from one of the files included from little_endian.h.

But not on Alpine:edge, that fails for all files where perf_event.h is
included but linux/stddef.h isn't include before that.

Adding the missing linux/stddef.h file where it breaks on Alpine:edge to
fix that, in all other distros, that is just a very small header anyway.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-9r1pifftxvuxms8l7ir73p5l@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>