ext: EXT4_KUNIT_TESTS should depend on EXT4_FS instead of selecting it

EXT4_KUNIT_TESTS selects EXT4_FS, thus enabling an optional feature the
user may not want to enable.  Fix this by making the test depend on
EXT4_FS instead.

Fixes: 1cbeab1b

 ("ext4: add kunit test for decoding extended timestamps")
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20210122110234.2825685-1-geert@linux-m68k.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>