Ignore safestringlib sourcefiles in coverage (#3632)

This is not our code, so we don't care about the coverage our tests generate
for it.

(cherry picked from commit 8deb805338)

.codecov.yml

@@ -11,6 +11,8 @@ coverage:
     - "src/backend/distributed/utils/citus_outfuncs.c"
     - "src/backend/distributed/deparser/ruleutils_*.c"
     - "src/include/distributed/citus_nodes.h"
+    - "src/backend/distributed/safeclib"
+    - "vendor"
 
   status:
     project: