Add entrypoint for welcome_message

The previous patch introducing the welcome_message code neglected to
add an entrypoint in jeepyb's setup.cfg, meaning the executable
was not created.

This patch adds the missing entry point

Change-Id: I38c83f962cb2f8c511e1e3b5ba4d7dac286bdd12
diff --git a/setup.cfg b/setup.cfg
index d71104c..a2a775d 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -28,3 +28,4 @@
     trivial-rebase = jeepyb.cmd.trivial_rebase:main
     update-blueprint = jeepyb.cmd.update_blueprint:main
     update-bug = jeepyb.cmd.update_bug:main
+    welcome-message = jeepyb.cmd.welcome_message:main