Address compiler warnings on some packaging platforms

I _believe_ these are from an RPM build; they've been sitting on my desktop for quite a while. At minimum, we should address them in the codebase. Ideally, we'd add the flags to our own builds to ensure we don't regress.

# Output from a `sort | count` run
```
3 /7.6.0.citus~git.20181101.3616939/src/backend/distributed/commands/multi_copy.c:1291:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
3 /7.6.0.citus~git.20181101.3616939/src/backend/distributed/utils/citus_clauses.c:77:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
1 /7.6.0.citus~git.20181101.3616939/src/backend/distributed/utils/ruleutils_10.c:4487:8: warning: this statement may fall through [-Wimplicit-fallthrough=]
1 /7.6.0.citus~git.20181101.3616939/src/backend/distributed/utils/ruleutils_11.c:4489:8: warning: this statement may fall through [-Wimplicit-fallthrough=]
3 /usr/include/postgresql/10/server/utils/elog.h:108:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
3 /usr/include/postgresql/11/server/utils/elog.h:108:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
3 /usr/include/postgresql/9.6/server/utils/elog.h:108:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
1 W: postgresql-9.6-citus-dbgsym: package-has-long-file-name 72 (81) > 80
24 configure: WARNING: unrecognized options: --disable-dependency-tracking
5 dpkg-buildpackage: warning: using a gain-root-command while being root
12 planner/query_colocation_checker.c:63: warning: 'colocatedJoinChecker.anchorAttributeEquivalences' may be used uninitialized in this function
12 planner/query_colocation_checker.c:63: warning: 'colocatedJoinChecker.subquery' may be used uninitialized in this function
12 planner/query_colocation_checker.c:63: warning: 'colocatedJoinChecker.subqueryPlannerRestriction' may be used uninitialized in this function
```