ubifs: Add missing dependency on GZIP

GZIP option can be manually de-selected when UBIFS is enabled. This cause
following compile error because ubifs calls gzip functions.

  /tmp/ccxVrh2c.ltrans1.ltrans.o: in function `gzip_decompress.lto_priv.566':
  <artificial>:(.text+0x768): undefined reference to `zunzip'
  collect2: error: ld returned 1 exit status
  make: *** [Makefile:1813: u-boot] Error 1

So add missing dependency on GZIP.

Signed-off-by: Pali Rohár <pali@kernel.org>