fix duplicated asm function

transport/internet/kcp/xor.go

@@ -1,3 +1,5 @@
+// +build !amd64
+
 package kcp
 
 // xorfwd performs XOR forwards in words, x[i] ^= x[i-4], i from 0 to len

transport/internet/kcp/xor_amd64.go

@@ -0,0 +1,4 @@
+package kcp
+
+func xorfwd(x []byte)
+func xorbkd(x []byte)