x86: entry_64.S - add missing ENDPROC

native_usergs_sysret64 is described as

	extern void native_usergs_sysret64(void)

so lets add ENDPROC here

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: heukelum@fastmail.fm
Cc: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>