perf tools: Release session and symbol resources on exit

So that we reduce the noise when looking for leaks using tools such as
valgrind.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/map.h

@@ -143,6 +143,7 @@ struct machine *machines__find(struct rb_root *self, pid_t pid);
 struct machine *machines__findnew(struct rb_root *self, pid_t pid);
 char *machine__mmap_name(struct machine *self, char *bf, size_t size);
 int machine__init(struct machine *self, const char *root_dir, pid_t pid);
+void machine__exit(struct machine *self);
 
 /*
  * Default guest kernel is defined by parameter --guestkallsyms