zephyr: fix include of system_timer.h

Above header was moved to another path.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/zephyr/main.c b/boot/zephyr/main.c
index d114522..b47da2b 100644
--- a/boot/zephyr/main.c
+++ b/boot/zephyr/main.c
@@ -19,7 +19,7 @@
 #include <gpio.h>
 #include <misc/__assert.h>
 #include <flash.h>
-#include <drivers/system_timer.h>
+#include <drivers/timer/system_timer.h>
 #include <usb/usb_device.h>
 #include <soc.h>