CMake: Install auth.conf.d directory

The missing auth.conf.d directory was an oversight, it should
be there.

LP: #1818996

CMakeLists.txt

@@ -243,6 +243,7 @@ endif()
 # Create our directories.
 install_empty_directories(
   ${CONF_DIR}/apt.conf.d
+  ${CONF_DIR}/auth.conf.d
   ${CONF_DIR}/preferences.d
   ${CONF_DIR}/sources.list.d
   ${CONF_DIR}/trusted.gpg.d