[3rdparty] Avoid macro redefinition in picojson (#153)

This PR adds a guard to avoid the macro redifinition of
`__STDC_FORMAT_MACROS` in 3rdparty picojson.

Author: MasterJH5574

3rdparty/picojson/picojson.h

@@ -29,8 +29,10 @@
 
 #ifndef PICOJSON_USE_INT64
 #define PICOJSON_USE_INT64
+#ifndef __STDC_FORMAT_MACROS
 #define __STDC_FORMAT_MACROS 1
 #endif
+#endif
 
 // If PICOJSON_USE_ORDERED_OBJECT is set, picojson uses object_with_ordered_keys, which maintains
 // the insertion order of keys, i.e. the order of keys in the json string.