Fix: don't define variables in headers

From Hollis Blanchard <hollis_blanchard@mentor.com>:
> Hi, I was adding an LTTng UST 2.0 tracepoint to an application that uses
> -warn-common (see http://www.math.utah.edu/docs/info/ld_2.html). I created
> a simple tracepoint, had lttng-gen-tp produce tracepoints.o, then linked
> that to the application, along with -llttng-ust. This results in some
> warnings:
>
> tracepoints.o: warning: common of `handle' overridden by definition
> /usr/local/lib/liblttng-ust.so: warning: defined here
> tracepoints.o: warning: common of `lttng_client_callbacks_overwrite' overridden
> +by definition
> /usr/local/lib/liblttng-ust.so: warning: defined here
> tracepoints.o: warning: common of `lttng_client_callbacks_discard' overridden by
> +definition
> /usr/local/lib/liblttng-ust.so: warning: defined here
> tracepoints.o: warning: common of `lttng_client_callbacks_metadata' overridden
> +by definition
> /usr/local/lib/liblttng-ust.so: warning: defined here
> /usr/local/lib/liblttng-ust-tracepoint.so.0: warning: multiple common of
> +`handle'
> tracepoints.o: warning: previous common is here
>
> This seems to be a valid warning. The LTTng UST headers contain
> definitions like this in include/lttng/ringbuffer-config.h:
> struct lttng_ust_shm_handle *handle;
>
> If two objects use that header, each will get a copy of "handle", right?

handle: This was meant to be a forward declaration of

  struct lttng_ust_shm_handle

so just removing the "*handle" part. This can be considered as a
cleanup (or a fix without actual runtime effect).

lttng_client_callbacks_*: if the cb values would have been used in the
consumer daemon, this would have caused an issue: these would be set to
NULL instead of the actual callback pointers. So in a way this is a fix,
but it does not have any runtime impact at this point.

Reported-by: Hollis Blanchard <hollis_blanchard@mentor.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>