emscripten: add main thread proxy for getting audio frequency

the rest of the calls were updated in aec86ba but one was missed

src/audio/emscripten/SDL_emscriptenaudio.c

diff --git a/src/audio/emscripten/SDL_emscriptenaudio.c b/src/audio/emscripten/SDL_emscriptenaudio.c
index dd55de8..a145b51 100644
--- a/src/audio/emscripten/SDL_emscriptenaudio.c
+++ b/src/audio/emscripten/SDL_emscriptenaudio.c
@@ -270,7 +270,7 @@
     this->hidden = (struct SDL_PrivateAudioData *)0x1;
 
     /* limit to native freq */
-    this->spec.freq = EM_ASM_INT({
+    this->spec.freq = MAIN_THREAD_EM_ASM_INT({
       var SDL2 = Module['SDL2'];
       return SDL2.audioContext.sampleRate;
     });