Fixed some compilation errors

[matlabbe]

Fixed some compilation errors and a linker error when including the library in another project.

[mpitropov]

I'm new to using this repo and this is the compile error on master:

Errors     << loam_velodyne:make /home/user/catkin_ws/logs/loam_velodyne/build.make.000.log
/home/user/catkin_ws/src/git_ros/loam_velodyne/src/lib/LaserMapping.cpp: In member function ‘virtual bool loam::LaserMapping::setup(ros::NodeHandle&, ros::NodeHandle&)’:
/home/user/catkin_ws/src/git_ros/loam_velodyne/src/lib/LaserMapping.cpp:149:28: error: ‘downSizeFilterMap’ was not declared in this scope
          downSizeFilterMap().setLeafSize(fParam, fParam, fParam);
                            ^
make[2]: *** [src/lib/CMakeFiles/loam.dir/LaserMapping.cpp.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [src/lib/CMakeFiles/loam.dir/all] Error 2
make: *** [all] Error 2
cd /home/user/catkin_ws/build/loam_velodyne; catkin build --get-env loam_velodyne | catkin env -si  /usr/bin/make --jobserver-fds=6,7 -j; cd -
...............................................................................
Failed     << loam_velodyne:make           [ Exited with code 2 ]              
Failed    <<< loam_velodyne                [ 44.0 seconds ]                    
[build] Summary: 0 of 1 packages succeeded.                                    
[build]   Ignored:   50 packages were skipped or are blacklisted.              
[build]   Warnings:  1 packages succeeded with warnings.                       
[build]   Abandoned: None.                                                     
[build]   Failed:    1 packages failed.                                        
[build] Runtime: 44.3 seconds total.                                           
[build] Note: Workspace packages have changed, please re-source setup files to use them.

I was able to compile using this branch. @laboshinl Would you be able to merge this in?