Fix: Compilation failure in session_not_found_error with GCC 4.8

Fix: Compilation failure in session_not_found_error with GCC 4.8

Observed issue
==============

When compiling with gcc 4.8.5, the compilation fails with the
following erorr:

```
session.hpp:577:2: error: function 'lttng::sessiond::exceptions::session_not_found_error::session_not_found_error(lttng::sessiond::exceptions::session_not_found_error&&)' defaulted on its first declaration with an exception-specification that differs from the implicit declaration 'lttng::sessiond::exceptions::session_not_found_error::session_not_found_error(lttng::sessiond::exceptions::session_not_found_error&&)'
session.hpp:577:2: error: function 'lttng::sessiond::exceptions::session_not_found_error::session_not_found_error(lttng::sessiond::exceptions::session_not_found_error&&)' defaulted on its first declaration with an exception-specification that differs from the implicit declaration 'lttng::sessiond::exceptions::session_not_found_error::session_not_found_error(lttng::sessiond::exceptions::session_not_found_error&&)'
```

Cause
=====

This is due a bug in GCC which is fixed as of GCC 5.0[1]

Solution
========

Do not explicitly define the move_assignable for
`lttng::sessiond::exceptions::session_not_found_error` as
`noexcept`. The function should be implicitly generated as `noexcept`.

Known drawbacks
===============

None.

References
==========
[1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59526

Change-Id: I3368633ce3b45627f2e67f7d2def361e662eec3d
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>