perf tools: Add missing object file to the python binding linkage list

In ac12f6764c50 ("perf tools: Implement branch_type event parameter") we
started using the parse_branch_str() function from one of the files used
in the python binding, which caused this entry in 'perf test' to fail:

# perf test -v python
16: Try 'import perf' in python, checking link problems :
--- start ---
test child forked, pid 16667
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ImportError: /tmp/build/perf/python/perf.so: undefined symbol:
parse_branch_str
test child finished with -1
---- end ----
Try 'import perf' in python, checking link problems: FAILED!
#

I must've commited some mistake when running 'perf test' to send the
pull request for the perf-core-for-mingo-20161024 tag, to have let this
regression to pass, sigh.

Just add tools/perf/util/parse-branch-options.c and switch from using
ui__warning(), that is not available in the python binding, use
pr_warning() instead, which is good enough for this case.

Now:

# perf test python
16: Try 'import perf' in python, checking link problems : Ok
#

Cc: Adrian Hunter <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Wang Nan <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Fixes: ac12f6764c50 ("perf tools: Implement branch_type event parameter")
Link: http://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/parse-branch-options.c

···
 		}
 		if (!br->name) {
 			ret = -1;
-			ui__warning("unknown branch filter %s,"
+			pr_warning("unknown branch filter %s,"
 				    " check man page\n", s);
 			goto error;
 		}

tools/perf/util/python-ext-sources

···
 util/util.c
 util/xyarray.c
 util/cgroup.c
+util/parse-branch-options.c
 util/rblist.c
 util/counts.c
 util/strlist.c