minor patch to lib/raid6/Makefile

[Steve Summit]

I've been doing some cross-compilation on my Mac, and one of
the little hiccups is that lib/raid6 won't build due to a small
command-line incompatibility between GNU awk and the one that
comes with MacOS.  Patch attached.  (I have nothing like commit
access; I'm hoping someone else can apply this.)
--- Makefile.orig	2012-11-26 16:40:34.000000000 -0500
+++ Makefile	2016-03-22 08:33:28.000000000 -0400
@@ -6,7 +6,7 @@
 hostprogs-y	+= mktables
 
 quiet_cmd_unroll = UNROLL  $@
-      cmd_unroll = $(AWK) -f$(srctree)/$(src)/unroll.awk -vN=$(UNROLL) \
+      cmd_unroll = $(AWK) -f $(srctree)/$(src)/unroll.awk -v N=$(UNROLL) \
                    < $< > $@ || ( rm -f $@ && exit 1 )
 
 ifeq ($(CONFIG_ALTIVEC),y)