selftests/ftrace: Convert poll to a gen_file

selftests/ftrace: Convert poll to a gen_file

Poll program is a helper to ftracetest, thus make it a
generic file and remove it from being run as a test.

Currently when executing tests using
    $ make run_tests
      CC       poll
    TAP version 13
    1..2
    # timeout set to 0
    # selftests: ftrace: poll
    # Error: Polling file is not specified
    not ok 1 selftests: ftrace: poll # exit=255

Fix this by using TEST_GEN_FILES to build the 'poll' binary as a helper
rather than as a test.

Fixes: 80c3e28528ff ("selftests/tracing: Add hist poll() support test")
Link: https://lore.kernel.org/r/20250409044632.363285-1-Ayush.jain3@amd.com
Signed-off-by: Ayush Jain <Ayush.jain3@amd.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>