package/network-manager: disable introspection

package/network-manager: disable introspection

Build will fail if gobject-introspection is built before network-manager
but python-gobject is not:

configure: error: "--enable-introspection aims to build the settings documentation. This requires GObject introspection for python (pygobject)

To avoid this build failure and because we don't need documentation,
just disable introspection

Fixes:
 - http://autobuild.buildroot.org/results/d3b1bc2fa7559e66465033c455176761d6e184d1

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>