Build fix: old gcc does not recognize hidden/shadowed enumeration as valid

The build fails on GCC < 6 with:

ust-registry.hpp: In constructor 'lttng::sessiond::ust::registry_typed_enum<MappingIntegerType>::registry_typed_enum(const char*, const lttng_ust_ctl_enum_entry*, size_t)':
ust-registry.hpp:111:45: error: 'lttng::sessiond::trace::integer_type::signedness' is not a class, namespace, or enumeration
       lttng::sessiond::trace::integer_type::signedness::SIGNED :

The same error occurs for stream_class::header_type.

This is due to a bug fixed in gcc 6:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60994

In both cases, the member is suffixed to disambiguate the reference to
the inner-enumeration.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Id0f2f98a147be589b9c70740660c7fb911dfc22c