Answer
The addition is not stereospecific because the Br can attack from either top or bottom.
Work Step by Step
After the double bond attacks the H in HBr, the carbocation at the most substituted carbon forms. The hybridization of this carbon is $sp^2$, which is planar. This planar carbon causes the non-stereoselectivity because the $Br^-$ can attack on either side, top or bottom.