tracing: append a comma to INIT_FTRACE_GRAPH

Impact: dont break future extensions of INIT_TASK

While not a problem right now, due to lack of a comma, build fails if
elements are appended to INIT_TASK() macro in development code:

 arch/x86/kernel/init_task.c:33: error: request for member `XXXXXXXXXX' in something not a structure or union
 arch/x86/kernel/init_task.c:33: error: initializer element is not constant
 arch/x86/kernel/init_task.c:33: error: (near initialization for `init_task.ret_stack')
 make[1]: *** [arch/x86/kernel/init_task.o] Error 1
 make: *** [arch/x86/kernel] Error 2

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: srostedt@redhat.com
LKML-Reference: <200904080505.n3855hcn017109@www262.sakura.ne.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>