tools: Add copy of perf_event.h to tools/include/linux/

We shouldn't use headers from the kernel sources directly, instead we
should use the system's headers or in cases where that isn't possible,
like with perf_event.h, where the introduction of kernel features such
as perf_event_attr.{write_backwards,sample_max_stack} and
PERF_EVENT_IOC_PAUSE_OUTPUT take some time to become available in
/usr/include/linux/perf_event.h we need a copy.

Do it and check for source code drift, emitting a warning when changes
are detected.

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: http://lkml.kernel.org/n/tip-v6aks5un3s5pehory6f42nrl@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.perf

@@ -345,6 +345,9 @@ export srctree OUTPUT RM CC LD AR CFLAGS V BISON FLEX AWK
 include $(srctree)/tools/build/Makefile.include
 
 $(PERF_IN): prepare FORCE
+	@(test -f ../../include/uapi/linux/perf_event.h && ( \
+        (diff -B ../include/uapi/linux/perf_event.h ../../include/uapi/linux/perf_event.h >/dev/null) \
+        || echo "Warning: tools/include/uapi/linux/perf_event.h differs from kernel" >&2 )) || true
 	$(Q)$(MAKE) $(build)=perf
 
 $(OUTPUT)perf: $(PERFLIBS) $(PERF_IN) $(LIBTRACEEVENT_DYNAMIC_LIST)