Add yarp/os/Things.h to YARP_OS_HDRS in CMakeLists.txt

The new header src/libYARP_OS/include/yarp/os/Things.h was added in yarp/os/all.h but not in the YARP_OS CMakeLists.txt. 
This is breaking any file external to yarp that is including the yarp/os/all.h header file.