fix building libqi without gtest
fix building libqi without gtest

Change-Id: I6cd6b1a4d876b6c0682cd67b9b7a51263036ced0
Reviewed-on: http://gerrit.aldebaran.lan:8080/6358
Reviewed-by: cgestes <ddb78fe00abb42f91254c16a0860e38eb0781789@aldebaran-robotics.com>
Tested-by: dmerejkowsky <1e4b13000fa8777df4cefd2fd8215187e3deb062@aldebaran-robotics.com>

bsautron/libqi,aldebaran/libqi,aldebaran/libqi,aldebaran/libqi,vbarbaresi/libqi