patch 9.1.0904: Vim9: copy-paste error in class_defining_member()

patch 9.1.0904: Vim9: copy-paste error in class_defining_member()

Problem:  Vim9: copy-paste error in class_defining_member()
Solution: use variable type VAR_CLASS instead (Zdenek Dohnal)

Found issue by OpenScanHub:
Error: COPY_PASTE_ERROR (CWE-398):
vim91/src/vim9class.c:3308: original: "VAR_OBJECT" looks like the
original copy.
vim91/src/vim9class.c:3316: copy_paste_error: "VAR_OBJECT" looks like a
copy-paste error.
vim91/src/vim9class.c:3316: remediation: Should it say "VAR_CLASS"
instead?
3314|             {
3315|                 cl_tmp = super;
3316|->               vartype = VAR_OBJECT;
3317|             }
3318|         }

closes: #16163

Signed-off-by: Zdenek Dohnal <zdohnal@redhat.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>