UBIFS: fix section mismatch

This patch fixes the following section mismatch:

WARNING: fs/ubifs/ubifs.o(.init.text+0xec): Section mismatch in reference from the function init_module() to the function .exit.text:ubifs_compressors_exit()

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>