Fix warning because of incompatible pointer type

ClosureTest.c: In function 'testThreadedClosureVrV':
ClosureTest.c:102:44: warning: cast between incompatible function types from 'void * (*)(void *)' to 'void (*)(void *)' [-Wcast-function-type]
  102 |     HANDLE hThread = (HANDLE) _beginthread((void (*)(void *))threadVrV, 0, &arg);
      |                                            ^

Fixes #745

spec/ffi/fixtures/ClosureTest.c

@@ -82,15 +82,23 @@ static void *
 threadVrV(void *arg)
 {
     struct ThreadVrV* t = (struct ThreadVrV *) arg;
-    
+
     int i;
     for (i = 0; i < t->count; i++) {
         (*t->closure)();
     }
-    
+
     return NULL;
 }
 
+#ifdef _WIN32
+static void
+threadVrV_win32(void *arg)
+{
+    threadVrV(arg);
+}
+#endif
+
 void testThreadedClosureVrV(void (*closure)(void), int n)
 {
 	struct ThreadVrV arg = {closure, n};
@@ -99,8 +107,8 @@ void testThreadedClosureVrV(void (*closure)(void), int n)
     pthread_create(&t, NULL, threadVrV, &arg);
     pthread_join(t, NULL);
 #else
-    HANDLE hThread = (HANDLE) _beginthread((void (*)(void *))threadVrV, 0, &arg);
-    WaitForSingleObject(hThread, INFINITE);	
+    HANDLE hThread = (HANDLE) _beginthread(threadVrV_win32, 0, &arg);
+    WaitForSingleObject(hThread, INFINITE);
 #endif
 }