avoid MSVC build error

```
Microsoft (R) Incremental Linker Version 14.00.24241.7
Copyright (C) Microsoft Corporation.  All rights reserved.
io.def : error LNK2001: unresolved external symbol luaopen_compat53_io
compat53/io.lib : fatal error LNK1120: 1 unresolved externals
Error: Build error: Failed compiling module compat53/io.dll
```

lprefix.h

@@ -242,21 +242,24 @@ static void createmeta (lua_State *L);
 #  undef LUA_FILEHANDLE
 #  define LUA_FILEHANDLE COMPAT53_LUA_PFILEHANDLE
 
+LUAMOD_API int luaopen_compat53_io (lua_State *L) {
+  luaL_Reg const funcs[] = {
+
 /* for PUC-Rio Lua 5.1 only */
 #  if defined(LUA_VERSION_NUM) && LUA_VERSION_NUM == 501 && !defined(LUA_JITLIBNAME)
 
-LUAMOD_API int luaopen_compat53_io (lua_State *L) {
-  luaL_Reg const funcs[] = {
     { "popen", io_popen },
     { "type", io_ptype },
+
+#  endif /* for PUC-Rio Lua 5.1 only */
+
     { NULL, NULL }
   };
   luaL_newlib(L, funcs);
   createmeta(L);
   return 1;
 }
 
-#  endif /* for PUC-Rio Lua 5.1 only */
 
 /* fake CLANG feature detection on other compilers */
 #  ifndef __has_attribute