Fix: Add missing .gitmodules and enable submodule checkout in CI

The OpenVX-cts directory was configured as a submodule but the .gitmodules
file was never committed. Additionally, the CI workflow was not initializing
submodules during checkout.

This commit:
- Adds .gitmodules with proper configuration for OpenVX-cts submodule
- Updates CI workflow to use submodules: recursive for checkout action

Fixes the CI failure where OpenVX-cts directory was empty during build.

Author: CI Bot

.github/workflows/openvx-conformance.yml

@@ -31,6 +31,7 @@ jobs:
       uses: actions/checkout@v4
       with:
         fetch-depth: 0
+        submodules: recursive
 
     - name: Debug directory structure
       run: |

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "OpenVX-cts"]
+	path = OpenVX-cts
+	url = https://github.com/KhronosGroup/OpenVX-cts.git