doc: xmlto output to stdout on a verbose build

When running 'make V=1' send the output of xmlto to stdout to help with
debugging command failures.

Change-Id: Ia974bb0ca9671129499b7966e8790a594ce034b2
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>