RFR: 8287745 jni/nullCaller/NullCallerTest.java fails with "exitValue = 1"

[Tim Prinzing]

Fixed JtregNativeJdk.gmk to include c++ libs for NullCallerTest

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Commit messages:
 - 8287745 Fix JtregNativeJdk.gmk to include c++ libs for NullCallerTest

Changes: https://git.openjdk.java.net/jdk/pull/9010/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9010&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287745
  Stats: 4 lines in 2 files changed: 1 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9010.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9010/head:pull/9010

PR: https://git.openjdk.java.net/jdk/pull/9010