zephyr: Add missing flash_map.h to sysflash.h

The sysflash.h defines FLASH_AREA_ macros using FIXED_PARTITION
macros that are provided by flash_map.h, but it does not include
the required header.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/zephyr/include/sysflash/sysflash.h b/boot/zephyr/include/sysflash/sysflash.h
index 8ad30e7..dd759e2 100644
--- a/boot/zephyr/include/sysflash/sysflash.h
+++ b/boot/zephyr/include/sysflash/sysflash.h
@@ -3,8 +3,9 @@
 #ifndef __SYSFLASH_H__
 #define __SYSFLASH_H__
 
-#include <zephyr/devicetree.h>
 #include <mcuboot_config/mcuboot_config.h>
+#include <zephyr/devicetree.h>
+#include <zephyr/storage/flash_map.h>
 
 #ifndef CONFIG_SINGLE_APPLICATION_SLOT