package/lxc: fix compilation with stub memfd_create

package/lxc: fix compilation with stub memfd_create

This fixes compilation issue in case memfd_create is a stub in libc.
Patch already merged in lxc master branch
(344b8ee293f4d3730a70a6ccaa03d7e4a516ae95).

Copy of the original commit log:

In case the internal memfd_create has to be used, make sure we don't
clash with the already existing memfd_create function from glibc.

This can happen if this glibc function is a stub. In this case, at
./configure time, the test for this function will return false, however
the declaration of that function is still available. This leads to
compilation errors.

Signed-off-by: Patrick Havelange <patrick.havelange@essensium.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>