Attempt to fix mingw compilation with std::variant

For some reason, it had no issue storing a value of a long long int in the variant with boost::variant.

src/scripting/lua_unit.cpp

@@ -469,7 +469,7 @@ static int impl_unit_set(lua_State *L)
 
 	if(strcmp(m, "upkeep") == 0) {
 		if(lua_isnumber(L, 3)) {
-			u.set_upkeep(luaL_checkinteger(L, 3));
+			u.set_upkeep(static_cast<int>(luaL_checkinteger(L, 3)));
 			return 0;
 		}
 		const char* v = luaL_checkstring(L, 3);