[foreign] RFR: tweak static type for LayoutType.VOID

[Maurizio Cimadamore]

Hi,
this simple patch tweaks the static type of LayoutType.ofVoid and 
NativeTypes.VOID to be LayoutType<Void> instead of LayoutType<?>, so 
that void pointer are more usable.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/void_layouttype/

Maurizio