Re-enable pr-head

The lack of pr-head is a problem. We can't get any CI results if
there's a merge conflict. Even with no conflict, pr-merge results
change when the target branch changes, and we don't have an easy way
to see if the pull request has had a passing job once pr-merge results
become stale. So turn on pr-head.

Explicitly mark the job as non-disabled by request.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Change-Id: I5cb3679842f2857c212d2ccfa4a4be29ba58f9eb
diff --git a/mbed-tls-pr-head.yaml b/mbed-tls-pr-head.yaml
index 479d71e..afca5b3 100644
--- a/mbed-tls-pr-head.yaml
+++ b/mbed-tls-pr-head.yaml
@@ -2,7 +2,7 @@
     name: mbed-tls-pr-head
     description: 'Job for testing mbed tls PR heads'
     project-type: multibranch
-    disabled: true
+    disabled: false
     prune-dead-branches: true
     days-to-keep: 60
     num-to-keep: 1000