tools.build: add sys/cdefs.h to SYSINCS, since lots of other headers use it

This is needed to let the legacy stage compile against newer versions of
sys/font.h, which transitively includes sys/cdefs.h, and requires the
new __nonstring macro from it.

Fixes: e2c93ed09f25
MFC after: 3 days

(cherry picked from commit 1c9ff80f06350fa44b7e50ce09ad665ab0082abb)