Remove compiler warnings

[20/67] Building CXX object CMakeFiles/petrack_core.dir/src/petrack.cpp.o
In file included from /home/tobias/development/petrack/petrack/src/petrack.cpp:29:
In file included from /home/tobias/development/petrack/petrack/./include/stereoItem.h:26:
In file included from /home/tobias/development/petrack/petrack/./include/petrack.h:43:
/home/tobias/development/petrack/petrack/./include/moCapController.h:31:1: warning: class 'SkeletonLine' was previously declared as a struct; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Wmismatched-tags]
class SkeletonLine;
^
/home/tobias/development/petrack/petrack/./include/skeletonTree.h:145:8: note: previous use is here
struct SkeletonLine{
       ^
/home/tobias/development/petrack/petrack/./include/moCapController.h:31:1: note: did you mean struct here?
class SkeletonLine;
^~~~~
struct
In file included from /home/tobias/development/petrack/petrack/src/petrack.cpp:49:
/home/tobias/development/petrack/petrack/./include/moCapItem.h:28:1: warning: class 'SegmentRenderData' was previously declared as a struct; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Wmismatched-tags]
class SegmentRenderData;
^
/home/tobias/development/petrack/petrack/./include/moCapController.h:34:8: note: previous use is here
struct SegmentRenderData
       ^
/home/tobias/development/petrack/petrack/./include/moCapItem.h:28:1: note: did you mean struct here?
class SegmentRenderData;
^~~~~
struct
2 warnings generated.