stand/fdt: Swap libfdt include order

libfdt.h should be included before fdt.h, as hinted at by all of libfdt/;
standard include order being libfdt.h, libfdt_env.h, fdt.h.

The current include order also causes problems when libfdt gets updated, as
fdt.h requires some definitions from libfdt.h.

Differential Revision:	https://reviews.freebsd.org/D13688

stand/fdt/fdt_loader_cmd.c

@@ -31,8 +31,8 @@
 __FBSDID("$FreeBSD$");
 
 #include <stand.h>
-#include <fdt.h>
 #include <libfdt.h>
+#include <fdt.h>
 #include <sys/param.h>
 #include <sys/linker.h>
 #include <machine/elf.h>