test: overlay: add missing include

Compiling the overlay unit test fails with odroid-c2_defconfig showing
errors like:

    test/overlay/cmd_ut_overlay.c:29:8:
    error: unknown type name ‘fdt32_t’

Add the missing include.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

test/overlay/cmd_ut_overlay.c

@@ -7,6 +7,7 @@
 #include <common.h>
 #include <command.h>
 #include <errno.h>
+#include <fdt_support.h>
 #include <malloc.h>
 
 #include <linux/sizes.h>