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
1 file changed