util/docker/coreboot-sdk: Install GNAT 12

For some reason GNAT 11 is not able to build GNAT 12, since there are
some Ada errors during the compilation. However, it works with GNAT 12.
So use GNAT 12 for the host toolchain instead.

Change-Id: If00a05a0c8564e624809268a12fae28261e380a2
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66924
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martin.roth@amd.corp-partner.google.com>

util/docker/coreboot-sdk/Dockerfile

@@ -35,7 +35,7 @@ RUN \
 		gawk \
 		gcc \
 		git \
-		gnat \
+		gnat-12 \
 		golang \
 		graphviz \
 		lcov \