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

(cherry picked from commit 6b5f70d1)