util/docker/coreboot-sdk: Install GNAT meta package instead gnat-12

The versions of both GCC and GNAT need to be in sync and the meta
package for GCC is already used. So use the meta package for GNAT as
well.

Change-Id: Ifcd6960731bc02c70a510e520b385ca300caf88f
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76086
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

util/docker/coreboot-sdk/Dockerfile

@@ -35,7 +35,7 @@ RUN \
 		gawk \
 		gcc \
 		git \
-		gnat-12 \
+		gnat \
 		golang \
 		graphicsmagick-imagemagick-compat \
 		graphviz \