MATLAB cannot do symbolic differentiation on an m-file. That would in general be impossible, since you could stick anything you wanted in there.
You have two choices:
1. Perform the differentiation in advance, using the sym tools. Then you can pass the derivative function also to your NR code.
2. Inside the NR code, use finite differencing to compute an approximation to the derivative. This is almost always adequate for Newton schemes, although care must be taken to get a good estimate, using an appropriate step size. Also, central differences are considerable more accurate, so use them whenever possible.