Fix Bug in thread.c: Only initialise and destroy mmap files table if VM is defined

src/threads/thread.c

@@ -15,11 +15,13 @@
 #include "threads/switch.h"
 #include "threads/synch.h"
 #include "threads/vaddr.h"
-#include "vm/page.h"
-#include "vm/mmap.h"
 #ifdef USERPROG
 #include "userprog/process.h"
 #include "userprog/syscall.h"
+#include "vm/page.h"
+#endif
+#ifdef VM
+#include "vm/mmap.h"
 #endif
 
 /* Random value for struct thread's `magic' member.
@@ -275,7 +277,9 @@ thread_create (const char *name, int priority,
     }
 #endif
 
+#ifdef VM
   mmap_init ();
+#endif
 
   /* Prepare thread for first run by initializing its stack.
      Do this atomically so intermediate values for the 'stack' 
@@ -397,7 +401,9 @@ thread_exit (void)
   process_exit ();
 #endif
 
+#ifdef VM
   mmap_destroy ();
+#endif
 
   /* Remove thread from all threads list, set our status to dying,
      and schedule another process.  That process will destroy us