Missing WINAPI for RegisterSystemThread.

diff --git a/windows/user.c b/windows/user.c
index 5829db7..3e2351e 100644
--- a/windows/user.c
+++ b/windows/user.c
@@ -568,7 +568,7 @@
 /***********************************************************************
  *	RegisterSystemThread	(USER32.435)
  */
-void RegisterSystemThread(DWORD flags, DWORD reserved)
+void WINAPI RegisterSystemThread(DWORD flags, DWORD reserved)
 {
 	FIXME(win32, "(%08lx, %08lx)\n", flags, reserved);
 }