cmd/spl.c: Include <libfdt.h> for fdt_totalsize

In order to be able to reliably use fdt_totalsize, we must have
<libfdt.h> included.

Fixes: 767cb74a00 ("cmd: spl: provide address and size of prepared FDT ...")
Signed-off-by: Tom Rini <trini@konsulko.com>

cmd/spl.c

@@ -8,6 +8,7 @@
 #include <common.h>
 #include <command.h>
 #include <cmd_spl.h>
+#include <libfdt.h>
 
 DECLARE_GLOBAL_DATA_PTR;