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

arch/x86/kernel/xsave.c

@@ -159,7 +159,7 @@ int save_i387_xstate(void __user *buf)
  * Restore the extended state if present. Otherwise, restore the FP/SSE
  * state.
  */
-int restore_user_xstate(void __user *buf)
+static int restore_user_xstate(void __user *buf)
 {
 	struct _fpx_sw_bytes fx_sw_user;
 	u64 mask;