[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.