cross-build: Avoid adding <sys/cdefs.h> to SYSINCS

Summary:
The sys/cdefs.h in src is incompatible with glibc's sys/cdefs.h so
cross-building broke when the former was added to SYSINCS in 1c9ff80f06. This
commit adds a guard around that to only do that when building on FreeBSD. This
should fix github CI.

Test Plan:
Ran buildkernel using tools/build/make.py on linux in github CI and
locally on FreeBSD to double check nothing broke.

Reviewed by: dim
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D54385