Small C++ header fixes.

TODO

@@ -24,3 +24,4 @@
   - valgrind check
   - -Werror no compiler warnings
   - test suite
+  - verify that all headers compile with C++.

src/upb_array.h

@@ -45,7 +45,7 @@ INLINE union upb_value_ptr upb_array_getelementptr(struct upb_array *arr,
 
 INLINE struct upb_array *upb_array_new(struct upb_msg_fielddef *f)
 {
-  struct upb_array *arr = malloc(sizeof(*arr));
+  struct upb_array *arr = (struct upb_array*)malloc(sizeof(*arr));
   upb_mmhead_init(&arr->mmhead);
   arr->elements._void = NULL;
   arr->len = 0;

src/upb_msg.h

@@ -141,7 +141,7 @@ INLINE struct google_protobuf_FieldDescriptorProto *upb_msg_field_descriptor(
  * counted reference. */
 INLINE struct upb_msg *upb_msg_new(struct upb_msgdef *md) {
   size_t size = md->size + offsetof(struct upb_msg, data);
-  struct upb_msg *msg = malloc(size);
+  struct upb_msg *msg = (struct upb_msg*)malloc(size);
   memset(msg, 0, size);
   upb_mmhead_init(&msg->mmhead);
   msg->def = md;