Missing include path for 'build-native-unix' task in lucene-misc module on Mac OS X [LUCENE-5346]

ant build-native-unix produces following error

```Java
[cc] In file included from /Users/stask/Projects/stask/lucene-4.5.1/misc/src/java/org/apache/lucene/store/NativePosixUtil.cpp:28:
[cc] /Library/Java/JavaVirtualMachines/jdk1.7.0_45.jdk/Contents/Home/include/jni.h:45:10: fatal error: 'jni_md.h' file not found
[cc] #include "jni_md.h"
[cc] ^
[cc] 1 error generated.
```

I added
```xml

```
to the build.xml of misc module to fix the issue.

---
Migrated from [LUCENE-5346](https://issues.apache.org/jira/browse/LUCENE-5346) by Stas Krichevsky
Environment:
```
Mac OS X 10.9
java version "1.7.0_45"
```

Inspect the misc module's build.xml and the build-native-unix task, then run ant build-native-unix on Mac OS X with the reported JDK layout. Done means the NativePosixUtil.cpp build finds jni_md.h and the task completes without the reported compiler error.

Written by the indexing model from the issue text.