lib/gnat: Use ADAFLAGS instead of CFLAGS

Commit 6d7564cdfe (Move -Wlogical-op into xcompile) introduced
GCC_ADAFLAGS_<arch> but forgot to use them for libgnat. Fix that.

Change-Id: Ia9079f01bb3c2a08296a3d3cc32fdf4ae5bb60c7
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33847
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/lib/gnat/Makefile.inc

@@ -24,7 +24,7 @@ ADAFLAGS_libgnat-$(1) := \
 	-gnatg \
 	-gnatpg \
 	-I$$(src)/lib/gnat/ \
-	$$(GCC_CFLAGS_$(1)) \
+	$$(GCC_ADAFLAGS_$(1)) \
 
 libgnat-$(1)-y += a-unccon.ads
 libgnat-$(1)-y += ada.ads