networkmanager: fix incorrect configure option

The wrong option was breaking the build with:

configure: error: invalid package name: systemdsystemunitdir=

Signed-off-by: Javier Viguera <javier.viguera@digi.com>
Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>