RFR: 8159366: Header guards missing for unittest.hpp

[Erik Helin]

Hi all,

this tiny patch adds header guards for test/native/unittest.hpp.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8159366

Webrev:
http://cr.openjdk.java.net/~ehelin/8159366/00/

Testing:
- JPRT

Thanks,
Erik