perf evsel: Introduce append_filter() method

perf evsel: Introduce append_filter() method

To allow building filters in evsel->filter, that will eventually be
applied via perf_evsel__apply_filter().

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-sjfoes3pycx7nlpmgedca13v@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>