Fix missing <endian.h> when cross-compiling on windows

This works because SDL checks <endian.h> iff SDL_BYTEORDER isn't defined.
6ad390fc50/include/SDL3/SDL_endian.h (L58-L60)

SDL3-CS/SDL3/ClangSharp/SDL_system.Linux.g.cs

@@ -0,0 +1,41 @@
+/*
+  <auto-generated/>
+  C# bindings for Simple DirectMedia Layer.
+  Original copyright notice of input files:
+
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2024 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+
+namespace SDL
+{
+    public static partial class SDL3
+    {
+        [DllImport("SDL3", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
+        [SupportedOSPlatform("Linux")]
+        public static extern int SDL_LinuxSetThreadPriority([NativeTypeName("Sint64")] long threadID, int priority);
+
+        [DllImport("SDL3", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
+        [SupportedOSPlatform("Linux")]
+        public static extern int SDL_LinuxSetThreadPriorityAndPolicy([NativeTypeName("Sint64")] long threadID, int sdlPriority, int schedPolicy);
+    }
+}

SDL3-CS/SDL3/SDL_system.Linux.rsp

@@ -0,0 +1,6 @@
+# Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+# See the LICENCE file in the repository root for full licence text.
+
+# Fix missing <endian.h> when cross-compiling on windows
+--define-macro
+SDL_BYTEORDER=SDL_LIL_ENDIAN