Makefile: Remove -gnatg from ADAFLAGS

The `-gnatg` flag (internal GNAT implementation mode) was accidentally
copied over from the flags we use to build libgnat.

Change-Id: I2e9c6d1d5b68168d1608187aad2eae5aead759f9
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/18780
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
1 file changed