GenericJMX plugin: fix indentation/bracket error

1e2ad9c0 contained a mistake which resulted in the following build
error. It seems to be a bad copy-paste when creating #1330 from the
patch in #1291.

```
./org/collectd/java/GenericJMXConfValue.java:535: error: variable tmp is already defined in constructor GenericJMXConfValue(OConfigItem)
          String tmp = getConfigString (child);
```