Either you use rpath to code the path to the shared object file into helloworld.so, or you put libmyprint.so into a standard library path (/usr/lib, /lib, directories in /etc/ld.so.conf, ...) and run ldconfig afterwards, or you set the LD_LIBRARY_PATH environment variable to the path where libmyprint.so lives. As an alternative you can also build libmyprint as a static library and link it into helloworld.so as a statically. That of course depend on the vendor of the library you want to bind as it'll only work if they provide a static library.

Cheers, Flo