Added RegisterSystemThread stub.

windows/user.c

diff --git a/windows/user.c b/windows/user.c
index 2bedecc..5829db7 100644
--- a/windows/user.c
+++ b/windows/user.c
@@ -564,3 +564,11 @@
 {	FIXME(win32,"(%08x,%08x),stub!\n",  hcur, id);
 	return TRUE;
 }
+
+/***********************************************************************
+ *	RegisterSystemThread	(USER32.435)
+ */
+void RegisterSystemThread(DWORD flags, DWORD reserved)
+{
+	FIXME(win32, "(%08lx, %08lx)\n", flags, reserved);
+}