src: fix non-linking with some compilers/linkers

src: fix non-linking with some compilers/linkers

We got a report that the library does not link within some
environments, giving messages like:

 undefined reference to `BusPathResolver::KDBUS_SYSTEM_BUS_PATH'

This is due to missing definition of static constexpr fields.
It is not necessary to give such definition when a field is initialized
inline since C++17, but this is compiled with C++11.

This commit adds definition of static constexpr fields.

Change-Id: Ia354f0e1aa5f1051571bf328fc654627f69c5dbd