doc/man/Makefile.am: handle vtracef(3) and vtracelog(3) correctly

doc/man/Makefile.am: handle vtracef(3) and vtracelog(3) correctly

This patch adds explicit grouped target rules to `doc/man/Makefile.am`
to handle the vtracef(3) and vtracelog(3) manual page generation
correctly.

This patch also adds `vtracef.3` and `vtracelog.3` to the list of files
to clean.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ia91db414693f1d336933a347ada13cfd430a9e53