util/cbfstool: unbreak compilation on FreeBSD

Compilation has been broken in commit I022468f6957415ae68a7a7e70428ae6f82d23b06
Adding a missing define solved this. See https://cgit.freebsd.org/src/tree/sys/sys/fcntl.h#n319

Signed-off-by: Idwer Vollering <vidwer@gmail.com>
Change-Id: I3433e4c9269880d3202dd494e5b2e962757a6b87
Reviewed-on: https://review.coreboot.org/c/coreboot/+/49314
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

util/cbfstool/partitioned_file.c

@@ -1,6 +1,8 @@
 /* read and write binary file "partitions" described by FMAP */
 /* SPDX-License-Identifier: GPL-2.0-only */
 
+#define __BSD_VISIBLE 1
+
 #include "partitioned_file.h"
 
 #include "cbfs_sections.h"