Assuming you are using Raspbian:
What appears to be happening is that the .config file when building raspbian's kernel and modules assigns 'y' values for the variables:
.... which supposedly means to integrate them into the kernel. Well, this either doesn't end up happening or usbip insists on using the modules anyway.
Long story short, what sort-of worked for me was to re-build the kernel and modules and re-install them:
... BUT before running the first 'make', open the .config file in the working directory, look for those variables mentioned above (they're clumped together) and set them to 'm' such that:
...then run 'make', 'make modules', etc.
The first 'make' takes about 10 hours.
If luck is on your side, you might be able to build and install just the modules which would be 'make modules' and 'make modules_install.' (don't copy the image) however if you're not lucky, the version of the modules and the version of your running kernel will not be the same and usbip likely will crash your kernel.
When changing settings for modules, the kernel image build (first 'make') doesn't have to be re-run, thankfully.