changed mangled method signature (#492)

Java 11 has changed the package name, so the name of the native peer method needed an according change.

src/peers/gov/nasa/jpf/vm/JPF_jdk_internal_misc_Unsafe.java

@@ -43,7 +43,7 @@
 public class JPF_jdk_internal_misc_Unsafe extends NativePeer {
 
   @MJI
-  public int getUnsafe____Lsun_misc_Unsafe_2 (MJIEnv env, int clsRef) {
+  public int getUnsafe____Ljdk_internal_misc_Unsafe_2 (MJIEnv env, int clsRef) {
     int objRef = env.getStaticReferenceField("jdk.internal.misc.Unsafe", "theUnsafe");
     return objRef;
   }