perf tools: Ditch rtrim(), use strim() from tools/lib

Cleaning up a bit more tools/perf/util/ by using things we got from the
kernel and have in tools/lib/

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-7hluuoveryoicvkclshzjf1k@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/python-ext-sources

@@ -16,6 +16,7 @@ util/namespaces.c
 ../lib/bitmap.c
 ../lib/find_bit.c
 ../lib/hweight.c
+../lib/string.c
 ../lib/vsprintf.c
 util/thread_map.c
 util/util.c