Update rs-motion.cpp
Update rs-motion.cpp

On instances where the first accelerometer frame arrives before the first gyro frame the "first" flag is set to false by process_accel which causes the first call to process_gryo to calculate the wrong ts value.
IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense