Add print to find

The `find` call in the Makefile doesn't actually output any `*.libsonnet` file due to the way `find` handles operators. This was discovered using GNU `find` on a Mac. From the manpages:

> Please  note  that -a when specified implicitly (for example by two tests appearing without an explicit operator between them) or explicitly has higher precedence than -o.  This means that find . -name afile -o -name bfile -print will never print afile.

A simple addition of `-print` to force the print fixes the issue.