tracing/syscalls: support for syscalls tracing on x86, fix

Impact: build fix

 kernel/built-in.o: In function `ftrace_syscall_exit':
 (.text+0x76667): undefined reference to `syscall_nr_to_meta'

ftrace.o is built:

obj-$(CONFIG_DYNAMIC_FTRACE)    += ftrace.o
obj-$(CONFIG_FUNCTION_GRAPH_TRACER) += ftrace.o

But now a CONFIG_FTRACE_SYSCALLS dependency is needed too.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
LKML-Reference: <1236401580-5758-3-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>