[common.h] Mark entry point as "unused".

This change defines a new, portable macro PYBIND11_MAYBE_UNUSED to
mark declarations as unused, and annotates the PYBIND11_MODULE entry
point with this attribute.

The purpose of this annotation is to facilitate dead code detection,
which might otherwise consider the module entry point function dead,
since it isn't otherwise used. (It is only used via FFI.)