selftests/ftrace: Fix to check required event file

The dynevent/test_duplicates.tc test case uses `syscalls/sys_enter_openat`
event for defining eprobe on it. Since this `syscalls` events depend on
CONFIG_FTRACE_SYSCALLS=y, if it is not set, the test will fail.

Add the event file to `required` line so that the test will return
`unsupported` result.

Fixes: 297e1dcd

 ("selftests/ftrace: Add selftest for testing duplicate eprobes and kprobes")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>