module.yml: Add snippet_root

This is the correct way for NCS snippets to be picked up automatically.
Previously, this was accomplished using a noup patch in sdk-zephyr.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>

Author: 57300

zephyr/module.yml

@@ -7,3 +7,4 @@ build:
     board_root: .
     dts_root: .
     module_ext_root: .
+    snippet_root: .