tracing/syscalls: Init print_fmt for syscall events

This is part of a patch set that removes the show_format method
in the ftrace event macros.

Add the print_fmt initialization to the syscall events.
The print_fmt is still not used, but will be in the follow up
patches.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
LKML-Reference: <4B273D41.609@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>