cmake: Add missing -DBUILDING_URBI_SDK to liburbi.
cmake: Add missing -DBUILDING_URBI_SDK to liburbi.

Change-Id: I1df3e1b0c4d7f580edff5edf67a520a6cea35fae
Reviewed-on: http://gerrit.aldebaran.lan:8080/15441
Reviewed-by: mnottale <e3ccf1b6558404e921a21fa09cebb1153b50ec36@aldebaran-robotics.com>
Tested-by: mnottale <e3ccf1b6558404e921a21fa09cebb1153b50ec36@aldebaran-robotics.com>

urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi