lockfile-util.c: fix build without F_OFD_SETLK

systemd fails to build on kernel without F_OFD_SETLK since
https://github.com/systemd/systemd/commit/9714c020fc4cda1823c2a77e3fd08aefa7d78b25

So put include missing_fcntl.h

Fixes:
 - http://autobuild.buildroot.org/results/699c078aa078240c6741da4dbd0871450ceeca92



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>