.redhat-ci.yml: add clang
.redhat-ci.yml: add clang

Clang has better detection for unused vars when using auto cleanup
functions. We should eventually just fold this back into the first
testsuite. But let's just turn it on for now, at least until it's
satisfied with the whole codebase.

Closes: #549
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree