zephyr: fix driver include paths

Zephyr will soon no longer add the drivers subdirectory of the include
hierarchy to the search path, so references to driver headers must
include the drivers/ prefix.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>
diff --git a/boot/zephyr/flash_map_extended.c b/boot/zephyr/flash_map_extended.c
index c50a1e9..b2de884 100644
--- a/boot/zephyr/flash_map_extended.c
+++ b/boot/zephyr/flash_map_extended.c
@@ -6,7 +6,7 @@
  */
 
 #include <zephyr.h>
-#include <flash.h>
+#include <drivers/flash.h>
 
 #include "target.h"