tracing: Prevent build warning: 'ftrace_graph_buf' defined but not used
tracing: Prevent build warning: 'ftrace_graph_buf' defined but not used

Prevent build warning when CONFIG_FUNCTION_GRAPH_TRACER is not set.

Signed-off-by: Lai Jiangshan <4c9eb49378914b743bdd32292c04df29b820d73e@cn.fujitsu.com>
LKML-Reference: <f17738752bf53a62b299c4fb943bfdcc12f89939@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <43232e92d70cc7aa53504ad0397085ee47bad87f@goodmis.org>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs