Enable PR Head job

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I0c47033fadf5a8e01767e05a67d4168053d77675
diff --git a/mbed-tls-pr-head.yaml b/mbed-tls-pr-head.yaml
index 1af2dd7..155da2d 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