Make sure Objective C files are built with the same flags as C files.

Fixes https://github.com/libsdl-org/SDL/issues/5707

CMakeLists.txt

@@ -3122,3 +3122,6 @@ endif()
 if(SDL_TEST)
   add_subdirectory(test)
 endif()
+
+##### Fix Objective C builds #####
+set(CMAKE_OBJC_FLAGS "${CMAKE_OBJC_FLAGS} ${CMAKE_C_FLAGS}")