PNP vs NPN input to LM7805 VR
You want a PNP type. That type sources current. 

If you need to use an existing NPN type you can add a high side switch, but at that point you'd perhaps be be better off using a regulator with an enable input as @Ignatio suggests. 

Here is one way to do the former: 







simulate this circuit &ndash; Schematic created using CircuitLab

The proper choice of R1/R2/Q2 depends on the current you intend to draw from U1.