Don't fail compilation on emscripten.

public/common/TracySystem.cpp

@@ -78,6 +78,9 @@ TRACY_API uint32_t GetThreadHandleImpl()
     return lwp_gettid();
 #elif defined __OpenBSD__
     return getthrid();
+#elif defined __EMSCRIPTEN__
+    // Not supported, but let it compile.
+    return 0;
 #else
     // To add support for a platform, retrieve and return the kernel thread identifier here.
     //