Fixed header dependencies to be fully compatible with the Windows
headers (with help from Dimitrie O. Paun).

diff --git a/scheduler/syslevel.c b/scheduler/syslevel.c
index 798133a..9ad2f51 100644
--- a/scheduler/syslevel.c
+++ b/scheduler/syslevel.c
@@ -20,10 +20,14 @@
 
 #include "config.h"
 
+#include <stdarg.h>
 #ifdef HAVE_UNISTD_H
 # include <unistd.h>
 #endif
 #include <sys/types.h>
+#include "windef.h"
+#include "winbase.h"
+#include "winreg.h"
 #include "winternl.h"
 #include "wine/winbase16.h"
 #include "syslevel.h"