[Metadata] Include picojson rather than forward declaring (#2118)

This PR fixes the picojson uses in MLC that conflicts with the latest
changes on the picojson side.

Author: MasterJH5574

cpp/metadata/model.h

@@ -5,20 +5,14 @@
 #ifndef MLC_LLM_CPP_MODEL_METADATA_H_
 #define MLC_LLM_CPP_MODEL_METADATA_H_
 
+#include <picojson.h>
 #include <tvm/runtime/container/shape_tuple.h>
 #include <tvm/runtime/container/string.h>
 #include <tvm/runtime/data_type.h>
 #include <tvm/runtime/module.h>
 
 #include <unordered_map>
 
-// Forward declare picojson's value, object and array
-namespace picojson {
-class value;
-using object = std::unordered_map<std::string, value>;
-using array = std::vector<value>;
-}  // namespace picojson
-
 namespace mlc {
 namespace llm {