Bump symbol versions

Both 5.5.0 and 5.6.0 introduce new symbols, but the former was
never even imported into the packaging repository, let alone
uploaded to Debian; 5.3.0 didn't introduce any new symbols.

debian/libvirt0.symbols

@@ -119,8 +119,9 @@ libvirt.so.0 libvirt0 #MINVER#
  *@LIBVIRT_4.10.0 4.10.0
  *@LIBVIRT_5.0.0 5.0.0
  *@LIBVIRT_5.2.0 5.2.0~rc1
- *@LIBVIRT_5.3.0 5.3.0
- *@LIBVIRT_PRIVATE_5.3.0 5.3.0
+ *@LIBVIRT_5.5.0 5.6.0
+ *@LIBVIRT_5.6.0 5.6.0
+ *@LIBVIRT_PRIVATE_5.6.0 5.6.0
 
 libvirt-qemu.so.0 libvirt0 #MINVER#
  *@LIBVIRT_QEMU_0.8.3 0.8.3
@@ -142,4 +143,4 @@ libvirt-admin.so.0 libvirt0 #MINVER#
  *@LIBVIRT_ADMIN_1.3.0 1.2.18
  *@LIBVIRT_ADMIN_2.0.0 2.0.0~rc1
  *@LIBVIRT_ADMIN_3.0.0 3.0.0
- *@LIBVIRT_ADMIN_PRIVATE_5.3.0 5.3.0
+ *@LIBVIRT_ADMIN_PRIVATE_5.6.0 5.6.0