vlib: fix mutable args

vlib/crypto/rc4/rc4.v

@@ -49,7 +49,7 @@ pub fn new_cipher(key []byte) ?Cipher {
 //
 // Deprecated: Reset can't guarantee that the key will be entirely removed from
 // the process's memory.
-pub fn (c &Cipher) reset() {
+pub fn (c mut Cipher) reset() {
 	for i in c.s {
 		c.s[i] = u32(0)
 	}
@@ -59,7 +59,7 @@ pub fn (c &Cipher) reset() {
 
 // xor_key_stream sets dst to the result of XORing src with the key stream.
 // Dst and src must overlap entirely or not at all.
-pub fn (c &Cipher) xor_key_stream(dst, src []byte) {
+pub fn (c mut Cipher) xor_key_stream(dst mut []byte, src []byte) {
 	if src.len == 0 {
 		return
 	}