scripts/sphinx-pre-install: fix out-of-tree build

Build of htmldocs fails for out-of-tree builds:

$ make V=1 O=~/build/kernel/ htmldocs
make -C /home/rppt/build/kernel -f /home/rppt/git/linux-docs/Makefile htmldocs
make[1]: Entering directory '/home/rppt/build/kernel'
make -f /home/rppt/git/linux-docs/scripts/Makefile.build obj=scripts/basic
rm -f .tmp_quiet_recordmcount
make -f /home/rppt/git/linux-docs/scripts/Makefile.build obj=Documentation htmldocs
Can't open Documentation/conf.py at /home/rppt/git/linux-docs/scripts/sphinx-pre-install line 230.
/home/rppt/git/linux-docs/Documentation/Makefile:80: recipe for target 'htmldocs' failed
make[2]: *** [htmldocs] Error 2

The scripts/sphinx-pre-install is trying to open files in the current
directory which is $KBUILD_OUTPUT rather than in $srctree.

Fix it.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>