docs: look for sphinx-pre-install in the source tree

Recent makefile changes included an invocation of
./scripts/sphinx-pre-install.  Unfortunately, that fails when a separate
build directory is in use with:

  /bin/bash: ./scripts/sphinx-pre-install: No such file or directory

Use $(srctree) to fully specify the location of this script.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/Makefile

@@ -23,7 +23,7 @@ ifeq ($(HAVE_SPHINX),0)
 .DEFAULT:
 	$(warning The '$(SPHINXBUILD)' command was not found. Make sure you have Sphinx installed and in PATH, or set the SPHINXBUILD make variable to point to the full path of the '$(SPHINXBUILD)' executable.)
 	@echo
-	@./scripts/sphinx-pre-install
+	@$(srctree)/scripts/sphinx-pre-install
 	@echo "  SKIP    Sphinx $@ target."
 
 else # HAVE_SPHINX