stub for IsSteamRunningOnSteamDeck

Index: src/utils.cpp
--- src/utils.cpp.orig
+++ src/utils.cpp
@@ -10,11 +10,18 @@ EXTERN int luasteam_getAppID(lua_State *L) {
     return 1;
 }
 
+// bool IsSteamRunningOnSteamDeck();
+// XXX: stub
+EXTERN int luasteam_isSteamRunningOnSteamDeck(lua_State *L) {
+    return 0;
+}
+
 namespace luasteam {
 
 void add_utils(lua_State *L) {
     lua_createtable(L, 0, 1);
     add_func(L, "getAppID", luasteam_getAppID);
+    add_func(L, "isSteamRunningOnSteamDeck", luasteam_isSteamRunningOnSteamDeck);
     lua_setfield(L, -2, "utils");
 }
 
