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/[e-mail]
Signed-off-by: Ayush Jain <[e-mail]>
Acked-by: Steven Rostedt (Google) <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/ftrace/Makefile

···
 TEST_FILES := test.d settings
 EXTRA_CLEAN := $(OUTPUT)/logs/*
 
-TEST_GEN_PROGS = poll
+TEST_GEN_FILES := poll
 
 include ../lib.mk