zephyr: add requirements.txt to package-managers in module.yml

This allows to install all requirements (Zephyr, mcuboot and pouch)
with:

  west packages pip --install

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>

Author: mniestroj

zephyr/module.yml

@@ -7,3 +7,7 @@ samples:
   - samples
 runners:
   - file: scripts/runners/__init__.py
+package-managers:
+  pip:
+    requirement-files:
+      - requirements.txt