See the following force diagram.

Vehicle is moving down the frictionless inclined surface so, it's acceleration is $g \sin \theta$ . Since vehicle is accelerating, a pseudo force $m(g \sin \theta)$ will act on bob of pendulum which cancel the $\sin \theta$ component of weight of the bob.

Hence net force on the bob is F net $= mg \cos \theta$ or net acceleration of the bob is $g_{\mathrm{eff}} = g \cos \theta$

... Time period $T = 2 \pi \sqrt{\frac{1}{g_{\text{eff}}}} = 2 \pi \sqrt{\frac{1}{g \cos \theta}}$