x11/libxshmfence: Pacify refined macro warnings

These macros should be defined in sys/mman.h. Actually, it was an old
Linux hack to work around header file inconsistency. For example,

https://manpages.ubuntu.com/manpages/bionic/man2/memfd_create.2.html
https://manpages.ubuntu.com/manpages/focal/man2/memfd_create.2.html

No functional change expected.