lxc: mark as not available in BR2_STATIC_LIBS configs

Even though lxc uses the autotools, it does not use libtool to build
its libraries, and does it manually. It forces building a shared
library. So let's comply with this decision, and mark lxc as not
available in BR2_STATIC_LIBS configurations.

Fixes:

  http://autobuild.buildroot.org/results/588/588dc28315a9114374f148bbc64c4ec3d7b4e295/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>