hdGp: Fix mismatched-tags warning for _ProcEntry

_ProcEntry had been defined as a struct but was subsequently
forward-declared as a class by TF_DECLARE_WEAK_PTRS. This
caused -Wmismatched-tags warnings when building on clang.

(Internal change: 2332206)

pxr/imaging/hdGp/generativeProceduralResolvingSceneIndex.h

@@ -107,8 +107,9 @@ class HdGpGenerativeProceduralResolvingSceneIndex
 
     static void _CombinePathArrays(const _DensePathSet &s, SdfPathVector *v);
 
-    struct _ProcEntry : public TfWeakBase
+    class _ProcEntry : public TfWeakBase
     {
+    public:
         enum State : unsigned char {
             StateUncooked = 0,
             StateDependenciesCooking,