Completed SORT_USES_MORE_RAM implementation

For the MORE_RAM option we need to buffer both the short and long
names, even though long names are sometimes redundant. Worst case, all
the names are max length. We can save some RAM by not storing these. We
could save more RAM by only storing the visible part of the long name.

Marlin/cardreader.h

@@ -73,6 +73,7 @@ private:
   uint16_t sort_count;
   uint8_t *sort_order;
   #if SORT_USES_MORE_RAM
+    char **sortshort;
     char **sortnames;
     uint8_t *isDir;
   #endif