styles: vocabularies: Yocto: add sstate

Add sstate as an accepted word to avoid errors when runnign make
stylecheck.

(From yocto-docs rev: 1c50726296e876747ea3f862729e953f025ce619)

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Reviewed-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/styles/config/vocabularies/Yocto/accept.txt

@@ -2,4 +2,5 @@ BitBake
 BSP
 crosstap
 OpenEmbedded
+sstate
 Yocto