extensions: fix parallel build
extensions: fix parallel build

Based on a similar fix from Debarshi Ray:
f1ab42c0c5332e47b33ecf905afd7fd6c2a344e8

https://bugzilla.gnome.org/show_bug.cgi?id=688132

Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets