fs/smart: fix visual studio Compiler Error C2016

D:\archer\code\nuttx\include\nuttx/fs/smart.h(118,1):
  error C2016: C requires that a struct or union has at least one member

Compiler error C2016: C requires that a struct or union has at least one member

Reference:
https://docs.microsoft.com/en-us/cpp/error-messages/compiler-errors-1/compiler-errors-c2001-through-c2099?view=msvc-170

Signed-off-by: chao.an <anchao@xiaomi.com>
Signed-off-by: chao an <anchao@xiaomi.com>

include/nuttx/fs/smart.h

@@ -114,6 +114,8 @@ struct smart_procfs_data_s
 #ifdef CONFIG_MTD_SMART_ERASE_DEBUG
   const uint16_t  *erasecounts;   /* Pointer to the erase counts array */
   uint16_t        erasesize;      /* Number of entries in the erase counts array */
+#else
+  uint8_t         __pad;
 #endif
 };
 #endif