Enable PR merge job

Currently, we're having Windows build nodes issues
if all mbedTLS jobs are enabled on the production [1].
Since PR merge job is relative lightweight compare to
PR Head job, enable it to see if it causes any unstable.

[1]: https://linaro.atlassian.net/browse/TFC-454?focusedCommentId=196472

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I20fbacefa5c6b0002d80dc02a979100146941821
diff --git a/mbed-tls-pr-merge.yaml b/mbed-tls-pr-merge.yaml
index beeccf4..e639086 100644
--- a/mbed-tls-pr-merge.yaml
+++ b/mbed-tls-pr-merge.yaml
@@ -2,7 +2,7 @@
     name: mbed-tls-pr-merge
     description: 'Job for testing PRs merged with the target branch head'
     project-type: multibranch
-    disabled: true
+    disabled: false
     prune-dead-branches: true
     days-to-keep: 60
     num-to-keep: 1000