prebuild script incorrectly attempting to extract tar/gz

[talentedbrute]

The prebuild script is attempt to extract a tar from the tar.gz dependencies: tar xf ../depends/assimp-4.0.1-mod.tar.gz -C ../ &

https://github.com/deptofdefense/AndroidTacticalAssaultKit-CIV/blob/master/scripts/prebuild.sh#L13

The fix is to add the z option to the tar: tar zxf ../depends/assimp-4.0.1-mod.tar.gz -C ../ &