systemd: Properly define the __NR_memfd_create macro for MIPS

This is an upstream patch which has been applied to the master branch
and I have adapted it to make it apply to the v216 version.

v217 is not fixed. This patch can be removed when we bump to v218.

Upstream commit:
  http://cgit.freedesktop.org/systemd/systemd/commit/?id=e6c019026b8cfd27a997e6e6ed1349f8f289b7e2

Fixes:
  http://autobuild.buildroot.net/results/596/59617a822fb2c8961881dd78a0dc8e41bdb14ede/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>