lxc: revive "drop-werror" patch

lxc: revive "drop-werror" patch

commit 5050a94ae9f448641e05ce29c931671ffdc62784 dropped patch that
remove '-Werror' from CFLAGS. However, it seems this patch is still
necessary:

  http://autobuild.buildroot.net/results/b1cbca6d0396863654b1d09ccc3163c5f6124ab8
  http://autobuild.buildroot.net/results/fcf2834ad74b95548d25dad2274704ea401f9665
  http://autobuild.buildroot.net/results/048be2fc702c9dba9ca4439ff687d71b30c10551
  http://autobuild.buildroot.net/results/939246c9a4f433dfd0dc414988f5957441b8e9b4

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>