cross-build: string.h uses mode_t, so we need sys/types.h

string.h defines strmode with a mode_t argument. POSIX states that one
must include sys/types.h to get mode_t, so do that here. This makes musl
happier. We know that sys/types.h will include sys/cdefs.h, so just
replace the latter with the former.

Co-authored-by: Val Packett <val@packett.cool>
Sponsored by: Netflix
Pull Request: https://github.com/freebsd/freebsd-src/pull/1066
Reviewed by: val_packett.cool
Differential Revision: https://reviews.freebsd.org/D45353