Unsafe increment of mutable static field

[sebbASF]

Test classes that increment static fields without sync:

jna/test/com/sun/jna/CallbacksTest.java

Line 85 in 6706361

public static int allocations = 0;

jna/test/com/sun/jna/ReturnTypesTest.java

Line 53 in 6706361

public static int allocations = 0;

jna/test/com/sun/jna/StructureTest.java

Line 533 in 6706361

public static int allocations = 0;

[matthiasblaesing]

I won't work on this, the unittests seem to work, unless proven probematic (not just theoretical) I don't see value in changes.