s390/diag: make __diag8c_tmp_amode31 static

Get rid of this sparse warning:

arch/s390/kernel/diag.c:69:29: warning: symbol '__diag8c_tmp_amode31' was not declared. Should it be static?

Fixes: fbaee746 ("s390/tty3270: add support for diag 8c")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/kernel/diag.c

@@ -66,7 +66,7 @@ static struct diag210 _diag210_tmp_amode31 __section(".amode31.data");
 struct diag210 __amode31_ref *__diag210_tmp_amode31 = &_diag210_tmp_amode31;
 
 static struct diag8c _diag8c_tmp_amode31 __section(".amode31.data");
-struct diag8c __amode31_ref *__diag8c_tmp_amode31 = &_diag8c_tmp_amode31;
+static struct diag8c __amode31_ref *__diag8c_tmp_amode31 = &_diag8c_tmp_amode31;
 
 static int show_diag_stat(struct seq_file *m, void *v)
 {