During cleanups, the logic checking if .config exists were dropped,
but removing it causes false-positives. So, re-add it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
---
 Documentation/Makefile | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Documentation/Makefile b/Documentation/Makefile
index cc4ee55c75ed..c60db1038c9c 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -42,8 +42,10 @@ FONTS_CONF_DENY_VF ?= $(HOME)/deny-vf
 # User-friendly check for sphinx-build
 HAVE_SPHINX := $(shell if which $(SPHINXBUILD) >/dev/null 2>&1; then echo 1; else echo 0; fi)
 
+ifneq ($(wildcard $(srctree)/.config),)
 ifeq ($(CONFIG_RUST),y)
-	RUSTDOC="--rustdoc"
+	RUSTDOC=--rustdoc
+endif
 endif
 
 ifeq ($(HAVE_SPHINX),0)
-- 
2.51.0