The time period of oscillation of a simple pendulum of length L...

Acceleration of vehicle down the incline will be $a = gsin\alpha$
In the frame of the vehicle, we have to apply pseudo force. The Force diagram is shown in figure.
Component of $mg$ along the inclined (i.e. $mgsin\alpha$) will balance the pseudo force. Therefore, ${g}_{eff} = g cos \alpha$


Hence,

$T = 2\pi\sqrt{\dfrac{L}{{g}_{eff}}} = 2\pi\sqrt{\dfrac{L}{g cos \alpha}}$.