SDL_mixer: CMake: Add @PACKAGE_INIT@ to the config template.

SDL Commits
1 
From d4bdddb2a624078f0f71e23fc33a386c7a0f4f49 Mon Sep 17 00:00:00 2001
From: Pierre Wendling <[EMAIL REDACTED]>
Date: Tue, 7 Mar 2023 16:13:47 -0500
Subject: [PATCH] CMake: Add @PACKAGE_INIT@ to the config template.

The config file was already configured with
`configure_package_config_file`.
---
 cmake/SDL3_mixerConfig.cmake.in | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/cmake/SDL3_mixerConfig.cmake.in b/cmake/SDL3_mixerConfig.cmake.in
index 534eb0c0..71e6d752 100644
--- a/cmake/SDL3_mixerConfig.cmake.in
+++ b/cmake/SDL3_mixerConfig.cmake.in
@@ -1,5 +1,7 @@
 # sdl3_mixer cmake project-config input for CMakeLists.txt script
 
+@PACKAGE_INIT@
+
 include(FeatureSummary)
 set_package_properties(SDL3_mixer PROPERTIES
     URL "https://www.libsdl.org/projects/SDL_mixer/"
@@ -117,3 +119,5 @@ if(NOT SDL3MIXER_VENDORED)
     set(CMAKE_MODULE_PATH "${_sdl_cmake_module_path}")
     unset(_sdl_cmake_module_path)
 endif()
+
+check_required_components(SDL3_mixer)