add import Cleaner for solving build fail

[benjioh5]

In some environment Cleaner makes build failure.
Cloud you check this patch?

$ gradle build -x test                                                   

> Task :core:compileJava
/Users/benjioh5/Coding/PIVXj/core/src/main/java/org/pivxj/store/WindowsMMapHack.java:34: error: cannot find symbol
        Cleaner cleaner = ((DirectBuffer) buffer).cleaner();
        ^
  symbol:   class Cleaner
  location: class WindowsMMapHack
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
Note: Some input files use unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 error

> Task :core:compileJava FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':core:compileJava'.
> Compilation failed; see the compiler error output for details.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

Deprecated Gradle features were used in this build, making it incompatible with Gradle 6.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/5.5.1/userguide/command_line_interface.html#sec:command_line_warnings

BUILD FAILED in 6s
3 actionable tasks: 1 executed, 2 up-to-date

[furszy]

hmm, It's because you are building with +1.9 java version, you will surely find lot of other issues compiling the code with an unsupported version. Use java 1.8.