app-emulation/lxc: disable -Werror, fixes build failure on gcc-7.1

On gcc-7.1 build fails as a result of both more warnings from gcc
(implicit fallthrough) and a format truncation gcc bug:
    https://gcc.gnu.org/PR78969#c4

Package-Manager: Portage-2.3.6, Repoman-2.3.2

app-emulation/lxc/lxc-2.0.7.ebuild

@@ -132,6 +132,7 @@ src_configure() {
 		--with-distro=gentoo \
 		--with-runtime-path=/run \
 		--disable-apparmor \
+		--disable-werror \
 		$(use_enable cgmanager) \
 		$(use_enable doc) \
 		$(use_enable examples) \