[syscall.inc] Adding default value to sys_open

src/syscalls.inc

@@ -34,7 +34,7 @@ O_NOCTTY   =  400o
 O_APPEND   = 2000o
 O_NONBLOCK = 4000o
 
-macro sys_open filename,flags,mode {
+macro sys_open filename*,flags*,mode=0 {
 	_syscall 2, filename, flags, mode
 }