perf symbols: Rename ->ignore to ->idle

perf symbols: Rename ->ignore to ->idle

Since this is the only use thus far, and this mechanism is in place for
a long time. To clarify why symbols should be skipped or treated
differently, name it for the only use it has.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-oqpf82x2svir611ry15paufd@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>