java: fix Java IsrCallback crash

The director now uses a GlobalRef instead of a WeakRef to the IsrCallback object

Signed-off-by: Petre Eftime <petre.p.eftime@intel.com>
Signed-off-by: Mihai Tudor Panu <mihai.tudor.panu@intel.com>

src/nrf24l01/javaupm_nrf24l01.i

@@ -2,6 +2,7 @@
 %include "../upm.i"
 
 %feature("director") Callback;
+SWIG_DIRECTOR_OWNED(Callback)
 
 %include "arrays_java.i";
 %apply signed char[] {uint8_t *};