Fix all -Wmissing-declarations warning instances

This fixes all the remaining -Wmissing-declarations warning occurences.
The changes either:

- Make functions static
- Remove unused functions
- Add declarations for functions that are meant to be exported in a
  shared object to override some other function (e.g.
  lttng_ust_clock_plugin_init).  There isn't really a better place for
  these declarations.

Change-Id: If08855b75bf44dfdcfbdd654c272474ad8ebef39
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>