Adding build dependencies

Prometheus charm no longer builds as it is missing
some dependencies

Change-Id: I6ae44d4dce922ed0bc589a0ddf18b4cc3ae75de0
Signed-off-by: Mark Beierl <mark.beierl@canonical.com>
diff --git a/installers/charm/prometheus/charmcraft.yaml b/installers/charm/prometheus/charmcraft.yaml
index babeda8..87d0463 100644
--- a/installers/charm/prometheus/charmcraft.yaml
+++ b/installers/charm/prometheus/charmcraft.yaml
@@ -35,5 +35,7 @@
 parts:
   charm:
     build-packages:
+      - cargo
       - git
       - libffi-dev
+      - rustc