Docs: Add sphinx to the documentation's "all" target

The all target really should build the expected output. At this point,
that means sphinx as well.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: Ibbceddd013605f5f7e11707480ae2a1e5d012a91
Reviewed-on: https://review.coreboot.org/c/coreboot/+/75498
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
diff --git a/Documentation/Makefile b/Documentation/Makefile
index 8552c6d..74e2bb8 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -8,7 +8,7 @@
 
 FIGS=codeflow.pdf hypertransport.pdf
 
-all: corebootPortingGuide.pdf
+all: sphinx corebootPortingGuide.pdf
 
 SVG2PDF=$(shell command -v svg2pdf)
 INKSCAPE=$(shell command -v inkscape)