cbfs: Restore 32-bit padding in cbfs_header

It was changed from a fixed size-1 array to a flexible array
in commit 242bac0e16 ("…: Use C99 flexible arrays") which resulted
in a change to the serialized format as the header size was no longer
the same.

That broke other tools that read CBFS files, like diffoscope
https://github.com/NixOS/nixpkgs/issues/256896

Change-Id: I4199dcc4823469c5986ac967a55b1c85cc62f780
Signed-off-by: Ivan Jager <aij+git@mrph.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78239
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Patrick Georgi <patrick@coreboot.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/commonlib/bsd/include/commonlib/bsd/cbfs_serialized.h

@@ -60,7 +60,7 @@ struct cbfs_header {
 	uint32_t align; /* fixed to 64 bytes */
 	uint32_t offset;
 	uint32_t architecture;
-	uint32_t pad[];
+	uint32_t pad[1];
 } __packed;
 
 /* this used to be flexible, but wasn't ever set to something different. */