Silence build warning on x86

Move the declaration of zephyr_flash_area_warn_on_open where all
targets can see it.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>

boot/zephyr/main.c

@@ -32,14 +32,14 @@ struct device *boot_flash_device;
 
 void os_heap_init(void);
 
+extern void zephyr_flash_area_warn_on_open(void);
+
 #if defined(CONFIG_ARM)
 struct arm_vector_table {
     uint32_t msp;
     uint32_t reset;
 };
 
-extern void zephyr_flash_area_warn_on_open(void);
-
 static void do_boot(struct boot_rsp *rsp)
 {
     struct arm_vector_table *vt;