[dsf] fix romfs build error in IAR (__weak should in start for variable definition)

components/dfs/filesystems/romfs/romfs.c

@@ -39,13 +39,13 @@ const static unsigned char _dummy_txt[] =
     0x74,0x68,0x69,0x73,0x20,0x69,0x73,0x20,0x61,0x20,0x66,0x69,0x6c,0x65,0x21,0x0d,0x0a,
 };
 
-const struct romfs_dirent RT_WEAK _root_dirent[] =
+RT_WEAK const struct romfs_dirent _root_dirent[] =
 {
     {ROMFS_DIRENT_DIR, "dummy", (rt_uint8_t *)_dummy, sizeof(_dummy)/sizeof(_dummy[0])},
     {ROMFS_DIRENT_FILE, "dummy.txt", _dummy_txt, sizeof(_dummy_txt)},
 };
 
-const struct romfs_dirent RT_WEAK romfs_root =
+RT_WEAK const struct romfs_dirent romfs_root =
 {
     ROMFS_DIRENT_DIR, "/", (rt_uint8_t *)_root_dirent, sizeof(_root_dirent)/sizeof(_root_dirent[0])
 };