c core: fix extra utf8 build conflict on windows

src/api/api.c

@@ -5,7 +5,7 @@ int luaopen_renderer(lua_State *L);
 int luaopen_regex(lua_State *L);
 int luaopen_process(lua_State *L);
 int luaopen_dirmonitor(lua_State* L);
-int luaopen_utf8(lua_State* L);
+int luaopen_utf8extra(lua_State* L);
 
 static const luaL_Reg libs[] = {
   { "system",     luaopen_system     },
@@ -13,7 +13,7 @@ static const luaL_Reg libs[] = {
   { "regex",      luaopen_regex      },
   { "process",    luaopen_process    },
   { "dirmonitor", luaopen_dirmonitor },
-  { "utf8",       luaopen_utf8       },
+  { "utf8extra",  luaopen_utf8extra  },
   { NULL, NULL }
 };