x86/retpoline: Move a NOENDBR annotation to the SRSO dummy return thunk

The linux-5.10-y backport of commit b377c66a ("x86/retpoline: Add
NOENDBR annotation to the SRSO dummy return thunk") misplaced the new
NOENDBR annotation, repeating the annotation on __x86_return_thunk,
rather than adding the annotation to the !CONFIG_CPU_SRSO version of
srso_alias_untrain_ret, as intended.

Move the annotation to the right place.

Fixes: 0bdc64e9 ("x86/retpoline: Add NOENDBR annotation to the SRSO dummy return thunk")
Reported-by: Greg Thelen <gthelen@google.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>