devel/libftdi1: prepare for boost 1.89

With boost 1.89, libboost_system, which is already an empty dummy
library will finally go away. Turn the boost dependency into a build
time dependency as no boost library is required at runtime.

Consumers of <ftdi.hpp> will have to depend on boost-libs at build time
to get the necessary headers, but a library dependency can be avoided
in many cases.

Approved by: bofh (maintainer)
MFH: 2025Q3