x86: xsave.c: restore_user_xstate should be static

Impact: cleanup, reduce kernel size a bit, avoid sparse warning

Fixes sparse warning:
arch/x86/kernel/xsave.c:162:5: warning: symbol 'restore_user_xstate' was not declared. Should it be static?

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>