Don't include main.c in libSDL_mixer (#21322)

This was broken when sdl2_mixer was updated recently in in #21013.

Fixes: #21321

Author: sbc100

test/test_browser.py

@@ -3405,7 +3405,7 @@ def test_sdl2_misc_via_object(self):
 
   @no_wasm64('SDL2 + wasm64')
   @parameterized({
-    'dash_s': (['-sUSE_SDL=2', '-sUSE_SDL_MIXER=2'],),
+    '': (['-sUSE_SDL=2', '-sUSE_SDL_MIXER=2'],),
     'dash_l': (['-lSDL2', '-lSDL2_mixer'],),
   })
   @requires_sound_hardware

tools/ports/sdl2_mixer.py

@@ -83,6 +83,7 @@ def create(final):
       exclude_files=[
         'playmus.c',
         'playwave.c',
+        'main.c',
       ],
       exclude_dirs=[
         'native_midi',