[PATCH] perf arm pmu: fix build error on MUSL libc

[Chanho Park]

__always_inline can cause build error on musl libc. The fix patch has
submitted but not merged yet[1]. To build perf tool with musl libc,
<linux/stddef.h> inclusion is necessary and it should be included before
perf_event.h.

from /usr/include/linux/byteorder/little_endian.h:13,
from /usr/include/asm/byteorder.h:23,
from tools/include/uapi/linux/perf_event.h:20,
from arch/arm64/util/../../arm/util/pmu.c:9:

/usr/include/linux/swab.h:171:8: error: unknown type name '__always_inline'
  171 | static __always_inline __u16 __swab16p(const __u16 *p)
      |        ^~~~~~~~~~~~~~~

[1]: https://lkml.org/lkml/2018/9/13/78
Cc: Mathieu Poirier <mathieu.poirier at linaro.org>
Cc: Suzuki K Poulose <suzuki.poulose at arm.com>
Cc: Mike Leach <mike.leach at linaro.org>
Cc: Leo Yan <leo.yan at linaro.org>
Cc: John Garry <john.garry at huawei.com>
Cc: Will Deacon <will at kernel.org>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Ingo Molnar <mingo at redhat.com>
Cc: Arnaldo Carvalho de Melo <acme at kernel.org>
Cc: Mark Rutland <mark.rutland at arm.com>
Cc: Jiri Olsa <jolsa at redhat.com>
Cc: Namhyung Kim <namhyung at kernel.org>
Cc: Khem Raj <raj.khem at gmail.com>
Signed-off-by: Chanho Park <chanho61.park at samsung.com>
---
 tools/perf/arch/arm/util/pmu.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/perf/arch/arm/util/pmu.c b/tools/perf/arch/arm/util/pmu.c
index bbc297a7e2e3..4c0357e8c0ab 100644
--- a/tools/perf/arch/arm/util/pmu.c
+++ b/tools/perf/arch/arm/util/pmu.c
@@ -6,6 +6,9 @@
 
 #include <string.h>
 #include <linux/coresight-pmu.h>
+#if !defined(__GLIBC__)
+#include <linux/stddef.h>
+#endif
 #include <linux/perf_event.h>
 #include <linux/string.h>
 
-- 
2.23.0