Apply Randrianasulu's fix to thread.cpp, fixes building in Termux, closes #4753.

src/thread.cpp

@@ -5,12 +5,16 @@
 #include <86box/plat.h>
 #include <86box/thread.h>
 
+#if !defined(__unix__)
+
 struct event_cpp11_t {
     std::condition_variable cond;
     std::mutex              mutex;
     bool                    state = false;
 };
 
+#endif
+
 extern "C" {
 
 thread_t *