util/cbfstool: Ensure that htole32 et al are visible when building

endian.h wasn't included (although it probably came in as an indirect
include) but in some header sets _XOPEN_SOURCE overrides _DEFAULT_SOURCE
whereas the latter is a super set of the former:

We should get the same things as with _XOPEN_SOURCE (such as memccpy for
which it has been defined) but also extra features like htole32.

Change-Id: Iaee7495b2ae64fdc719ae0879ea95fe7df286212
Signed-off-by: Patrick Georgi <patrick@coreboot.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59891
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Rob Barnes <robbarnes@google.com>

util/cbfstool/flashmap/fmap.c

@@ -1,8 +1,7 @@
 /* SPDX-License-Identifier: BSD-3-Clause or GPL-2.0-only */
 
-#define _XOPEN_SOURCE 700
-
 #include <ctype.h>
+#include <endian.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>