Move .gitignore from drivers/md to lib/raid6

Another missing bit of the raid6 -> /lib move.
Reported-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/.gitignore

-mktables
-raid6altivec*.c
-raid6int*.c
-raid6tables.c

lib/raid6/.gitignore

+mktables
+altivec*.c
+int*.c
+tables.c