libmtp: fix build error with glibc 2.23

With glibc 2.23, libmtp fails to build.
This CL fixes that.

BUG=chromium:616824
TEST=libmtp builds with glibc 2.23

Original CL: https://codereview.chromium.org/2190433002/
Original Author: yunlian@chromium.org

Review URL: https://codereview.chromium.org/2195013002 .
2 files changed