Start building rousette

Also, given that the CI-built images were already peaking at 395 MB used
in rootfs, add some breathing space.

Change-Id: Ia1b3485364ba0f3fa23b77c867ff51a9d8cefacc
diff --git a/.gitmodules b/.gitmodules
index 669c57f..599aae4 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -16,3 +16,6 @@
 [submodule "submodules/lldp-systemd-networkd-sysrepo"]
 	path = submodules/lldp-systemd-networkd-sysrepo
 	url = ../lldp-systemd-networkd-sysrepo
+[submodule "submodules/rousette"]
+	path = submodules/rousette
+	url = ../rousette