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>

scripts/sphinx-pre-install

@@ -5,8 +5,11 @@ use strict;
 # Copyright (c) 2017-2019 Mauro Carvalho Chehab <mchehab@kernel.org>
 #
 
-my $conf = "Documentation/conf.py";
-my $requirement_file = "Documentation/sphinx/requirements.txt";
+my $prefix = "./";
+$prefix = "$ENV{'srctree'}/" if ($ENV{'srctree'});
+
+my $conf = $prefix . "Documentation/conf.py";
+my $requirement_file = $prefix . "Documentation/sphinx/requirements.txt";
 my $virtenv_prefix = "sphinx_";
 
 #