wibox.layout.align: Give middle widget full height

I amended some change to commit 8560de597c3a1f349 which made the align layout's
middle widget really centered instead of being way too wide. However, this also
shrunk the widget on the "other" axis, too. This commit fixes that up.

A big "sorry" to Lukáš for breaking his patch.

Signed-off-by: Uli Schlachter <psychon@znc.in>