lxc: fix build with static libcap and shared gnutls

Fixes:
 - http://autobuild.buildroot.net/results/b655d6853c25a195df28d91512b3ffb6c654fc90



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Thomas:
 - add Upstream-status
 - use proper numbering]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>