efi_selftest: fix device tree unit test

Include libfdt.h was moved by commit b08c8c48 ("libfdt: move headers to
<linux/libfdt.h> and <linux/libfdt_env.h>")

Fixes: e236200c7fa6 ("efi_selftest: check installation of the
       device tree")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>

lib/efi_selftest/efi_selftest_fdt.c

@@ -12,7 +12,7 @@
  */
 
 #include <efi_selftest.h>
-#include <libfdt.h>
+#include <linux/libfdt.h>
 
 static struct efi_boot_services *boottime;
 static const char *fdt;