fix compilation on OSX Yosemite

Change-Id: Ic39e4797d9b85c00a3d696add203e27acb008790
Reviewed-on: http://gerrit.aldebaran.lan/48319
Reviewed-by: dmerejkowsky <dmerejkowsky@aldebaran-robotics.com>
Tested-by: dmerejkowsky <dmerejkowsky@aldebaran-robotics.com>