build: Avoid useless rebuilds of network plugins
build: Avoid useless rebuilds of network plugins

The dependencies on directories should be always order-only (which
means, after a pipe character). This change just slipped through my
review.

monstermunchkin/rkt,monstermunchkin/rkt,shobhit85/rkt,lucasallan/rkt,jjlakis/rkt,sgotti/rkt,ybubnov/rkt,yifan-gu/rocket,jonboulle/rkt,Wilybald/rkt,nhlfr/rkt,jonboulle/rkt,joshix/rkt,achanda/rkt,intelsdi-x/rkt,lucasallan/rkt,coreos/rkt,Wilybald/rkt,Shashwatsh/rkt,achanda/rkt,kinvolk/rkt,sstabellini/rkt,Shashwatsh/rkt,joshix/rkt,jjlakis/rkt,achanda/rkt,coreos/rkt,akshaykarle/rkt,euank/rkt,alban/rkt,joshix/rkt,dgonyeo/rkt,Wilybald/rkt,rkt/rkt,coreos/rkt,joshix/rkt,kinvolk/rkt,jonboulle/rkt,s-urbaniak/rkt,yifan-gu/rkt,yifan-gu/rocket,yifan-gu/rocket,kinvolk/rkt,euank/rkt,Wilybald/rkt,sgotti/rkt,cab105/rkt,dgonyeo/rkt,monstermunchkin/rkt,shobhit85/rkt,cab105/rkt,euank/rkt,jakon89/rkt,lucasallan/rkt,ybubnov/rkt,Shashwatsh/rkt,squeed/rkt,s-urbaniak/rkt,yifan-gu/rkt,jakon89/rkt,jjlakis/rkt,alban/rkt,sstabellini/rkt,sstabellini/rkt,nhlfr/rkt,cab105/rkt,alban/rkt,sgotti/rkt,rkt/rkt,jakon89/rkt,nhlfr/rkt,rkt/rkt,intelsdi-x/rkt,s-urbaniak/rkt,dgonyeo/rkt,intelsdi-x/rkt,rkt/rkt,akshaykarle/rkt,kinvolk/rkt,akshaykarle/rkt,squeed/rkt,shobhit85/rkt,squeed/rkt,yifan-gu/rkt,lucasallan/rkt,ybubnov/rkt