CI: Set DEBUG=-DDEBUG_STRICT=1 in GitHub Actions

This was missing when migrating from Travis.
diff --git a/.github/workflows/compliance.yml b/.github/workflows/compliance.yml
index d778ee4..9f2bec2 100644
--- a/.github/workflows/compliance.yml
+++ b/.github/workflows/compliance.yml
@@ -30,6 +30,7 @@
           ERR=1 \
           TARGET=${{ matrix.TARGET }} \
           CC=${{ matrix.CC }} \
+          DEBUG=-DDEBUG_STRICT=1 \
           USE_OPENSSL=1
         sudo make install
     - name: Show HAProxy version