Build fix: g++ < 7.1 mishandles namespaces of specializations

Due to a bug in g++ < 7.1, these specializations must be explicitly
enclosed in the namespaces rather than using the usual
`namespace::namespace::function` notation.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56480.

We already use this work-around in a number of places.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ie0e6a3c4dcfb4bc1581c2d273d73ea3e65b2ccd6