Add missing 'libvirt_lxc_api' variable in pkg-config file

The 'libvirt_lxc_api' variable is intended to allow apps to
query the location of the API XML file for libvirt_lxc.so
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

libvirt.pc.in

@@ -6,6 +6,7 @@ datarootdir=@datarootdir@
 
 libvirt_api=@datadir@/libvirt/api/libvirt-api.xml
 libvirt_qemu_api=@datadir@/libvirt/api/libvirt-qemu-api.xml
+libvirt_lxc_api=@datadir@/libvirt/api/libvirt-lxc-api.xml
 
 Name: libvirt
 Version: @VERSION@