Public functions in C should be declared before they are defined, and
often compilers warn you if you haven't:
```
../cogl/cogl/cogl-trace.c:237:1: warning: no previous prototype for
‘cogl_set_tracing_enabled_on_thread_with_fd’ [-Wmissing-prototypes]
cogl_set_tracing_enabled_on_thread_with_fd (void *data,
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../cogl/cogl/cogl-trace.c:245:1: warning: no previous prototype for
‘cogl_set_tracing_enabled_on_thread’ [-Wmissing-prototypes]
cogl_set_tracing_enabled_on_thread (void *data,
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../cogl/cogl/cogl-trace.c:253:1: warning: no previous prototype for
‘cogl_set_tracing_disabled_on_thread’ [-Wmissing-prototypes]
cogl_set_tracing_disabled_on_thread (void *data)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```
In this case the function declarations were not included because
`HAVE_TRACING` isn't defined. But we should still include `cogl-trace.h`
because it handles the case of `HAVE_TRACING` not being defined and
correctly still declares the functions defined in `cogl-trace.c`.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/650
d7d75dd8e7