Add helper functions to initialise the memory-mapped files table and counter

src/vm/mmap.h

@@ -2,6 +2,7 @@
 #define VM_MMAP_H
 
 #include <hash.h>
+#include "threads/thread.h"
 
 /* A mapping identifier type. */
 typedef unsigned mapid_t;
@@ -15,4 +16,6 @@ struct mmap_entry {
   struct hash_elem elem; /* An elem for the hash table. */
 };
 
+bool mmap_init (void);
+
 #endif /* vm/mmap.h */