make getPointerType() const, for good measure

c++/src/capnp/layout.c++

@@ -2423,7 +2423,7 @@ void PointerBuilder::clear() {
   memset(pointer, 0, sizeof(WirePointer));
 }
 
-PointerType PointerBuilder::getPointerType() {
+PointerType PointerBuilder::getPointerType() const {
   if(pointer->isNull()) {
     return PointerType::NULL_;
   } else {

c++/src/capnp/layout.h

@@ -318,7 +318,7 @@ public:
   // location.
 
   inline bool isNull() { return getPointerType() == PointerType::NULL_; }
-  PointerType getPointerType();
+  PointerType getPointerType() const;
 
   StructBuilder getStruct(StructSize size, const word* defaultValue);
   ListBuilder getList(ElementSize elementSize, const word* defaultValue);