Reinstate nanopb, since sel4test requires it

Change-Id: I38815413217e8fca331f2ae1c6f3b6fcc5a12411

manifest.xml

diff --git a/manifest.xml b/manifest.xml
index 4af0320..9bb4517 100644
--- a/manifest.xml
+++ b/manifest.xml
@@ -51,6 +51,7 @@
   <project name="3p/sel4/sel4_tools"             path="cantrip/tools/seL4" />
   <project name="3p/sel4proj/sel4runtime"        path="cantrip/projects/sel4runtime" />
   <project name="3p/sel4/util_libs"              path="cantrip/projects/util_libs" />
+  <project name="3p/nanopb/nanopb"               path="cantrip/tools/nanopb" />
   <!-- END cantrip components -->
 
   <!-- BEGIN ML toolchain-->