Since commit 7198c6ab
the ./mtr --embedded tests would fail to start innodb_plugin
because of an undefined reference to the symbol wsrep_log().

Let us define a stub for that function. The embedded server
is never built WITH_WSREP, but there are no separate storage
engine builds for the embedded server. Hence, by default,
the dynamic InnoDB storage engine plugin would be built WITH_WSREP
and it would fail to load into the embedded server library due to
a reference to the undefined symbol.