Fix a bug in `OctreePointCloudAdjacency::computeNeighbors()`

The `x`, `y`, and `z` fields of `OctreeKey` are unsigned integers, so
in some cases unsigned integer underflow occurs and the tree is queried
for non-existing leaves with huge keys. Due to the way `findLeaf()` is
implemented, such queries occasionally succeed and return some random
leaves.
Updated the fix for compute neighbors octree key bug
Added header for PCL_ERROR macro
fixed gpu/utils/CMakeLists missing ')'
Merge pull request #464 from AlexSchwank/master

fixed gpu/utils/CMakeLists missing ')'
Merge pull request #455 from jpapon/octree_key_fix

Fix bug in OctreePointCloudAdjacency::computeNeighbors()