selftests/ftrace: Fix BTFARG testcase to check fprobe is enabled correctly

Since the dynevent/add_remove_btfarg.tc test case forgets to ensure that
fprobe is enabled for some structure field access tests which uses the
fprobe, it fails if CONFIG_FPROBE=n or CONFIG_FPROBE_EVENTS=n.
Fixes it to ensure the fprobe events are supported.

Fixes: d892d3d3

 ("selftests/ftrace: Add BTF fields access testcases")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>