lib/raid6: fix unnecessary rebuild of vpermxor*.c

The following four files are every time rebuilt:

  UNROLL  lib/raid6/vpermxor1.c
  UNROLL  lib/raid6/vpermxor2.c
  UNROLL  lib/raid6/vpermxor4.c
  UNROLL  lib/raid6/vpermxor8.c

Fix the suffixes in the targets.

Fixes: 72ad2107

 ("lib/raid6: refactor unroll rules with pattern rules")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>