builtin: fix building vinix (missing C.strerror)

vlib/builtin/builtin.c.v

@@ -107,6 +107,7 @@ pub fn c_error_number_str(errnum int) string {
 	$if freestanding {
 		err_msg = 'error $errnum'
 	} $else {
+		$if !vinix {
 			c_msg := C.strerror(errnum)
 			err_msg = string{
 				str: &byte(c_msg)
@@ -114,6 +115,7 @@ pub fn c_error_number_str(errnum int) string {
 				is_lit: 1
 			}
 		}
+	}
 	return err_msg
 }