wsrep_certification_rules: Define as a weak global symbol.
While there are separate _embedded.a for statically
linked storage engine plugins, there is only one ha_innodb.so
which is supposed to work with both values of WITH_WSREP.
The merge from 10.0-galera introduced a reference to a global
variable that is only defined when the server is built WITH_WSREP.
We must define that symbol as weak global, so that when
a dynamically linked InnoDB or XtraDB is used with the embedded
server (which never includes write-set replication patches),
the variable will be read as 0, instead of causing a failure to
load the InnoDB or XtraDB plugin.
