Add missing dependency find to package config

refer to <https://cmake.org/cmake/help/latest/guide/importing-exporting/index.html#creating-a-package-configuration-file>

Config.cmake.in

@@ -1,5 +1,10 @@
 @PACKAGE_INIT@
 
+include(CMakeFindDependencyMacro)
+
 if(NOT TARGET IrrlichtMt::IrrlichtMt)
+	if(@USE_SDL2@)
+		find_dependency(SDL2)
+	endif()
 	include("${CMAKE_CURRENT_LIST_DIR}/IrrlichtMtTargets.cmake")
 endif()