of/overlay: Directly include idr.h

The overlay code uses IDRs but does not explicitly include the header
providing the interface, instead relying on an implicit inclusion. Make
the dependency explicit to avoid potential future build issues if the
implicit inclusion goes away.
Signed-off-by: NMark Brown <broonie@kernel.org>
Signed-off-by: NRob Herring <robh@kernel.org>

drivers/of/overlay.c

@@ -19,6 +19,7 @@
 #include <linux/string.h>
 #include <linux/slab.h>
 #include <linux/err.h>
+#include <linux/idr.h>
 
 #include "of_private.h"