cross-build: Move fcntl.h workaround to Linux specific file

Move the cdefs.h inclusion to the Linux specific file. Either place will
work, but it makes more sense to do it in the Linux specific file since
it's a Linux specific workaround.

Fixes: 4300e053618f
Suggested by: arichards, jrtc27
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D45373