alpha: fix build after vmlinux.lds.S cleanup

Add include to get missing THREAD_SIZE definition
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Tim Abbott <tabbott@ksplice.com>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Richard Henderson <rth@twiddle.net>

arch/alpha/kernel/vmlinux.lds.S

 #include <asm-generic/vmlinux.lds.h>
+#include <asm/thread_info.h>
 #include <asm/page.h>
 
 OUTPUT_FORMAT("elf64-alpha")