lib/raid6: fix sparse warnings in recovery functions

Make the recovery functions static to fix the following sparse warnings:

lib/raid6/recov.c:25:6: warning: symbol 'raid6_2data_recov_intx1' was
not declared. Should it be static?
lib/raid6/recov.c:69:6: warning: symbol 'raid6_datap_recov_intx1' was
not declared. Should it be static?
lib/raid6/recov_ssse3.c:22:6: warning: symbol 'raid6_2data_recov_ssse3'
was not declared. Should it be static?
lib/raid6/recov_ssse3.c:197:6: warning: symbol 'raid6_datap_recov_ssse3'
was not declared. Should it be static?

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Jim Kukunas <james.t.kukunas@linux.intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>