perf evsel: Introduce perf_evsel__prev() method

perf evsel: Introduce perf_evsel__prev() method

Just one use so far, on the hists browser, for completeness since there
we use perf_evlist__{first,last} and perf_evsel__next() for handling the
TAB and UNTAB keys.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-d09l4lejp5427enuf3igpckw@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>