Sync (make same) the offsetof macro definition in include/ with the
definition of the same in sys/sys/. The problem was discovered while
working on implementing a new C11 gets_s() for libc. (The new gets_s()
requires rsize_t found in include/stddef.h.) The solution to sync the two
definitions was suggested by ed@ while discussing D12667.

Suggested by: ed
MFC after: 2 weeks