Namaste means : Main tumhara manya karta hoon. I offer my respect/acceptance [not worship] unto you.
Refer to Point no.100 in ‘Arya uddeshyaratnamala’ by Rishi Dayanand, Rs 4/- http://www.vedicbooks.com
Thus it is not shirk. Shirk means associating the name of mortals with God , like u do when u say la ilaha illala mohammed-u-rasulallah.