Add missing dependency to the fedora-flags test

Other packages that use this test won't have clang pre-installed, so we
need to explicitly add the dependency.

tests/fedora-flags/main.fmf

@@ -2,6 +2,7 @@ summary: fedora-flags
 test: "$WITH_SCL ./test.sh"
 require:
   - annobin
+  - clang
   - redhat-rpm-config
 adjust:
   - because: s390x does not have epel repo which is required to provide rpm macros