ath79/mikrotik: add missing kernel config symbol for 5.4

The UBIFS_FS_ZSTD is exposed when UBIFS is enabled.

Signed-off-by: Roger Pueyo Centelles <[email protected]>
[adjust commit title]
Signed-off-by: Adrian Schmutzler <[email protected]>

target/linux/ath79/mikrotik/config-default

@@ -32,6 +32,7 @@ CONFIG_UBIFS_FS=y
 CONFIG_UBIFS_FS_ADVANCED_COMPR=y
 CONFIG_UBIFS_FS_LZO=y
 CONFIG_UBIFS_FS_ZLIB=y
+# CONFIG_UBIFS_FS_ZSTD is not set
 CONFIG_WATCHDOG_CORE=y
 CONFIG_ZLIB_DEFLATE=y
 CONFIG_ZLIB_INFLATE=y