Add C decl block to threading_primitives.h (#26437)

Forgot to add this as part of #26433

Author: sbc100

system/include/emscripten/threading_primitives.h

@@ -5,8 +5,14 @@
  * found in the LICENSE file.
  */
 
+#pragma once
+
 #include <emscripten/atomic.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 // Similar to emscripten_async_wait_callback_t but with a volatile first
 // argument.
 typedef void (*emscripten_async_wait_volatile_callback_t)(volatile void* address, uint32_t value, ATOMICS_WAIT_RESULT_T waitResult, void* userData);
@@ -180,3 +186,7 @@ ATOMICS_WAIT_TOKEN_T emscripten_condvar_wait_async(emscripten_condvar_t * _Nonnu
 // Pass numWaitersToSignal == EMSCRIPTEN_NOTIFY_ALL_WAITERS to wake all waiters
 // ("broadcast" operation).
 void emscripten_condvar_signal(emscripten_condvar_t * _Nonnull condvar, int64_t numWaitersToSignal);
+
+#ifdef __cplusplus
+}
+#endif