FROMLIST: setlocalversion: use ${objtree}/include/config/auto.conf

setlocalversion reads include/config/auto.conf, which is located below
$(objtree) with commit 214c0eea ("kbuild: add $(objtree)/ prefix to
some in-kernel build artifacts").

To be consistent, the setlocalversion script should use
${objtree}/include/config/auto.conf as well.

Signed-off-by: HONG Yifan <elsk@google.com>
Link: https://lore.kernel.org/lkml/20250318005900.2256921-1-elsk@google.com/
Bug: 402239869
Change-Id: Iffd89cad27100eacd48341b3812fbdbf800c6be4