build: Fix build with gtk-doc=true

Commit a76b28bd moved the dbus-interfaces to the
dbus-interfaces directory without changing the
path for docs build as well, resulting in a fail
of the build when building with gtk_doc=true.

So change the path in the build accordingly to
ensure that the build does not fail when building
with gtk_doc=true.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/236

docs/reference/shell/meson.build

@@ -24,7 +24,7 @@ ifaces = [
 ]
 foreach iface : ifaces
   custom_target(iface[0] + ' docs',
-    input: '../../../data/@0@.xml'.format(iface[0]),
+    input: '../../../data/dbus-interfaces/@0@.xml'.format(iface[0]),
     output: 'doc-gen-' + iface[1],
     command: [
       'gdbus-codegen',