tree.sh: add missing colon at the end of trees()

this is because when using chained commands at the end
of functions, sometimes you have to explicitly terminate
the line.

the way i do it in this patch is common across the
build system, to mitigate this sh quirk.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/tree.sh

@@ -64,7 +64,7 @@ trees()
 
 	targets="$*"
 	cmd="build_targets $targets"
-	singletree "$project" && cmd="build_project"
+	singletree "$project" && cmd="build_project"; :
 }
 
 build_project()