enable name mangling

Recent changes should have made gpfsl work with name mangling, so let's have CI ensure that this remains the case.

.gitlab-ci.yml

@@ -34,6 +34,7 @@ build-coq.8.17.0:
     OPAM_PINS: "coq version 8.17.0"
     DENY_WARNINGS: "1"
     OPAM_PKG: "1"
+    MANGLE_NAMES: "1"
   tags:
   - fp-timing