perf probe: Do not show @plt function by default

Fix the perf-probe --functions option do not show the PLT
stub symbols (*@plt) by default.

-----
$ ./perf probe -x /usr/lib64/libc-2.33.so -F | head
a64l
abort
abs
accept
accept4
access
acct
addmntent
addseverity
adjtime
-----

Reported-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Masami Hiramatsu <mhriamat@kernel.org>
Acked-by: Thomas Richter <tmricht@linux.ibm.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Stefan Liebler <stli@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Link: http://lore.kernel.org/lkml/162532653450.393143.12621329879630677469.stgit@devnote2
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>