selftests/ftrace: multiple_kprobes: Also check for support

The multiple_kprobes test case fails to check for KPROBE_EVENT support.
Add the check to prevent a false test result.

Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>