perf tools: Move ltrim() to util/string.c

As we have ltrim() implementation in builtin-script.c move it to the
more generic location of util/string.c so that it can be used from other
places.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1358845787-1350-14-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>