docs: fix out-of-tree build

When compiling NetworkManager with --enable-gtk-doc outside the
source tree, the generated documents are slightly different from
those generated in tree. This patch fixes that missed COPYING file in
$(top_builddir) and adds $(top_builddir)/libnm-util to DOC_SOURCE_DIR.

https://bugzilla.gnome.org/show_bug.cgi?id=742139

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>