vp9_rtcd_defs: fix vp9_avg_8x8 declaration

vp9_avg_8x8 does not depend on x86inc, fixes 32-bit OS X build

Change-Id: I709b874ea84bf57c8cdb5ac7d43eecc6b8c1a2dd

vp9/common/vp9_rtcd_defs.pl

@@ -1112,7 +1112,7 @@ add_proto qw/unsigned int vp9_get_mb_ss/, "const int16_t *";
 specialize qw/vp9_get_mb_ss/, "$sse2_x86inc";
 
 add_proto qw/unsigned int vp9_avg_8x8/, "const uint8_t *, int p";
-specialize qw/vp9_avg_8x8/, "$sse2_x86inc";
+specialize qw/vp9_avg_8x8 sse2/;
 
 # ENCODEMB INVOKE