You haven't installed the module websocket. Edit modules.lst file in the source code directory and add websocket to include_modules variable. If you don't have modules.lst, just do:
make cfg
Alternative is to do:
make cfg include_modules="websocket"
By default, the build system for kamailio compiles and install only the modules that have the same dependencies as the core of the application. For websocket you need to install libunistring and openssl (libssl) devel packages.