Satisfy Leo's thirst for structs and extlibs

include/groundvm.h

@@ -31,6 +31,7 @@ typedef enum ListAccessStatus {
 
 struct GroundValue;
 struct GroundFunction;
+struct GroundStruct;
 
 struct List;
 
@@ -58,8 +59,7 @@ typedef struct GroundError {
  */
 typedef struct GroundValue {
     GroundValueType type;
-    // This line seems to be causing issues with extlibs
-    // struct GroundStruct* customType;
+    struct GroundStruct* customType;
     union {
         int64_t intVal;
         double doubleVal;