src/lxc/meson.build: fix build without apparmor

src/lxc/meson.build: fix build without apparmor

Don't build lsm/apparmor.c if apparmor is explicitly disabled by the
user to avoid the following build failure with gcc 4.8:

/home/buildroot/autobuild/run/instance-3/output-1/host/arm-buildroot-linux-gnueabi/sysroot/usr/include/bits/fcntl2.h: In function '__apparmor_process_label_open.isra.0':
/home/buildroot/autobuild/run/instance-3/output-1/host/arm-buildroot-linux-gnueabi/sysroot/usr/include/bits/fcntl2.h:50:24: error: call to '__open_missing_mode' declared with attribute error: open with O_CREAT in second argument needs 3 arguments
    __open_missing_mode ();
                        ^

Fixes:
 - http://autobuild.buildroot.org/results/c9f05ad264543adf429badb99310905427092772

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>