Silence -Wimplicit-fallthrough in sysv_shmem.c.
Silence -Wimplicit-fallthrough in sysv_shmem.c.

Commit 2f932f71d9f2963bbd201129d7b971c8f5f077fd added code that elicits
a warning on buildfarm member flaviventris.  Back-patch to 9.4, like
that commit.

Reported by Andres Freund.

Discussion: e7ed770a404cb9c86d1bc3750666f47d758c6aa6@alap3.anarazel.de

adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb