Core: fix compiler warning in libusb_setlocale()

Hello,

A small patch for:

strerror.c: In function 'libusb_setlocale':
strerror.c:148: warning: comparison between signed and unsigned
strerror.c:152: warning: comparison between signed and unsigned

--
 Dr. Ludovic Rousseau

From a4144845845cd0a06fb9074ba2d6669ece3a5b1a Mon Sep 17 00:00:00 2001
From: Ludovic Rousseau <ludovic.rousseau+github@gmail.com>
Date: Wed, 19 Jun 2013 13:16:31 +0200
Subject: [PATCH] Core: fix compiler warning in libusb_setlocale()

strerror.c: In function 'libusb_setlocale':
strerror.c:148: warning: comparison between signed and unsigned
strerror.c:152: warning: comparison between signed and unsigned

Signed-off-by: Hans de Goede <hdegoede@redhat.com>