tracing: Initialize ret in syscall_enter_define_fields()

If syscall_enter_define_fields() is called on a system call with no
arguments, the return code variable "ret" will never get initialized.
Initialize it to zero.

Fixes: 04ae87a5 ("ftrace: Rework event_create_dir()")
Reported-by: Qian Cai <cai@lca.pw>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/0FA8C6E3-D9F5-416D-A1B0-5E4CD583A101@lca.pw