[meson] add boost as dependency

I setup a new development environment and emper did not compile because emper::io::Stats use the circular_buffer provided by boost. Boost was not installed and our build-system failed to detect it.

This change adds the header-only boost dependency to emper. https://mesonbuild.com/Dependencies.html#boost The header-only dependency is enough to build emper default configuration.

When linking against boost is required we use the 'modules' karg.