tools: Adopt __packed from kernel sources

To have a more compact way to ask the compiler to not insert alignment
paddings in a struct, making tools/ look more like kernel source code.

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-byp46nr7hsxvvyc9oupfb40q@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/include/linux/compiler-gcc.h

@@ -22,6 +22,8 @@
 
 #define  noinline	__attribute__((noinline))
 
+#define __packed	__attribute__((packed))
+
 #define __noreturn	__attribute__((noreturn))
 
 #define __printf(a, b)	__attribute__((format(printf, a, b)))