ci: Add verification of generated source code

The transition from travis to github CI dropped the check for verifying if the
source code generator was updated if it needed to for the changes.

Change-Id: I53fabfe14f42fe1c9899b2938f5e7b8073362954
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 2e3ece7..635511e 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -61,6 +61,9 @@
             - name: Build the loader
               run: make -C build
 
+            - name: Verify generated source files
+              run: python scripts/generate_source.py --verify external/Vulkan-Headers/registry
+
     windows:
         runs-on: ${{matrix.os}}
 
@@ -97,6 +100,9 @@
             - name: Build the loader
               run: cmake --build ./build --config ${{matrix.config}}
 
+            - name: Verify generated source files
+              run: python scripts/generate_source.py --verify external/Vulkan-Headers/registry
+
     mac:
         runs-on: macos-latest