testffmpeg: treat Khronos headers as system headers

test/CMakeLists.txt

@@ -247,7 +247,7 @@ if(FFmpeg_FOUND AND LIBAVUTIL_AVFRAME_HAS_CH_LAYOUT)
         target_link_libraries(testffmpeg PRIVATE OpenGL::EGL)
         target_compile_definitions(testffmpeg PRIVATE HAVE_EGL)
     endif()
-    target_include_directories(testffmpeg BEFORE PRIVATE ${SDL3_SOURCE_DIR}/src/video/khronos)
+    target_include_directories(testffmpeg SYSTEM BEFORE PRIVATE ${SDL3_SOURCE_DIR}/src/video/khronos)
     target_link_libraries(testffmpeg PRIVATE ${FFMPEG_LIBRARIES})
 else()
     message(STATUS "Can't find ffmpeg 5.1.3 or newer, skipping testffmpeg")