zephyr: fix do_boot() warning on ARM

Commit 7238f51 ("zephyr: add support for x86 booting method")
introduced a backend-specific do_boot() function. On ARM, this casts
to an undefined type 'struct vector_table*'.  Fix this by using the
'struct arm_vector_table*' introduced in that patch.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
diff --git a/boot/zephyr/main.c b/boot/zephyr/main.c
index 4e79f0a..885f667 100644
--- a/boot/zephyr/main.c
+++ b/boot/zephyr/main.c
@@ -49,8 +49,8 @@
 	 * consecutively. Manually set the stack pointer and jump into the
 	 * reset vector
 	 */
-	vt = (struct vector_table *)(rsp->br_image_addr +
-				     rsp->br_hdr->ih_hdr_size);
+	vt = (struct arm_vector_table *)(rsp->br_image_addr +
+					 rsp->br_hdr->ih_hdr_size);
 	irq_lock();
 	_MspSet(vt->msp);
 	((void (*)(void))vt->reset)();