Move mj_loadModel to xml_api.cc.

PiperOrigin-RevId: 635748029
Change-Id: Ie4c7d34c9738e5497ec700089d4dba780ba7d5ed

src/xml/xml_api.h

@@ -39,6 +39,9 @@ MJAPI void mj_freeLastXML(void);
 MJAPI int mj_printSchema(const char* filename, char* buffer, int buffer_sz,
                          int flg_html, int flg_pad);
 
+// load model from binary MJB file
+// if vfs is not NULL, look up file in vfs before reading from disk
+MJAPI mjModel* mj_loadModel(const char* filename, const mjVFS* vfs);
 
 #ifdef __cplusplus
 }