Add valve to water filter supply line
Looks like that water line to the filter is 1/4 tubing.   Go to local big box hardware store. int he plumbing section there will be a selection on little bags containing either brass or plastic parts, valves, compression fittings etc. 

Find either a compression fittingGood) or a push-in valve(Better) for the tubing size.  Either way make sure cuts to tubing a clean and square, no angles use a knife and piece of scrap wood to cut...not a pair of scissors.

Which ever you chose follow instructions to install correctly!