build: fix build without HAVE_CAPNG

Otherwise, a build may fail with:

lxc/lxc_conatiner.c: In function 'lxcContainerDropCapabilities':
lxc/lxc_container.c:1662:46: error: unused parameter 'keepReboot' [-Werror=unused-parameter]

* src/lxc/lxc_container.c (lxcContainerDropCapabilities): Mark
parameter unused.

src/lxc/lxc_container.c

@@ -1659,7 +1659,7 @@ static int lxcContainerSetupMounts(virDomainDefPtr vmDef,
  * It removes some capabilities that could be dangerous to
  * host system, since they are not currently "containerized"
  */
-static int lxcContainerDropCapabilities(bool keepReboot)
+static int lxcContainerDropCapabilities(bool keepReboot ATTRIBUTE_UNUSED)
 {
 #if HAVE_CAPNG
     int ret;