x86/srso: Add a forgotten NOENDBR annotation

Fix:

  vmlinux.o: warning: objtool: .export_symbol+0x29e40: data relocation to !ENDBR: srso_untrain_ret_alias+0x0

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>