force mut a := ..., do not allow mut a = ...

compiler/fn.v

@@ -531,7 +531,7 @@ fn (p mut Parser) fn_call(f Fn, method_ph int, receiver_var, receiver_type strin
 		if !receiver.is_mut && receiver_type.contains('*') {
 			method_call += '*'
 		}
-		mut cast = ''
+		mut cast := ''
 		// Method returns (void*) => cast it to int, string, user etc
 		// number := *(int*)numbers.first()
 		if f.typ == 'void*' {