perf data: Fix sentinel setting for data_cmds array

perf data: Fix sentinel setting for data_cmds array

The recent new patch "perf tools: Add new 'perf data' command" (commit
c6ebd919 in acme's git repo perf/core) has caused a building error when
compiling the source code of perf:

 cc1: warnings being treated as errors
 builtin-data.c:89: error: missing initializer
 builtin-data.c:89: error: (near initialization for ‘data_cmds[1].summary’)
 make[2]: *** [builtin-data.o] Error 1
 make[2]: *** Waiting for unfinished jobs....
   LD       bench/perf-in.o
   LD       tests/perf-in.o
 make[1]: *** [perf-in.o] Error 2
 make: *** [all] Error 2

This patch fixes the building error above.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1425038026-27604-1-git-send-email-yunlong.song@huawei.com
[ .name == NULL ends the loop, use it instead of seting all fields to NULL ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>