btrfs-progs: Fix -Wsuggest-attribute=format warning

Add __attribute__ ((format (printf, 4, 0))) to fix the vprintf calling
function.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

string-table.c

@@ -48,6 +48,7 @@ struct string_table *table_create(int columns, int rows)
  * '>' the text is right aligned. If fmt is equal to '=' the text will
  * be replaced by a '=====' dimensioned on the basis of the column width
  */
+__attribute__ ((format (printf, 4, 0)))
 char *table_vprintf(struct string_table *tab, int column, int row,
 			  const char *fmt, va_list ap)
 {