when the wedge is moved with accl^n a the body of mass m experieces a psuedo force ma towards opposite direction of the motion of wedge (as shown in figure).

Now resolving the forces as shown in figure...

we get ( for the body to remain at rest)

ma.cos = mg.sin

or a = g.tan ____________(1)

This is the accl^n to be provided to the wedge so that the block doesnot fall.

So again from figure we get that the force which the block gives the incline perpendicularly = ma.sin + mg.cos 

This force = force given by incline on the block....( from Newton's 3rd Law)

So reqd. force = ma.sin + mg.cos

                     = mg.tan.sin + mg.cos .........[ by (1) ]

                     = mg.(sin²/cos) + mg.cos

                     = mg.(sin² + cos²) / cos

                     = mg / cos...