Fix CMake flag so socket path becomes again correct (Closes: #911880)

Upstream apparently changed the flag name. To keep up with previous
functionality we need to reflect this change in d/rules.

debian/rules

@@ -15,7 +15,7 @@ override_dh_auto_configure:
 	-DPLUGINDIR_DEB=mariadb3 \
 	-DWITH_MYSQLCOMPAT=ON \
 	-DWITH_EXTERNAL_ZLIB:BOOL=ON \
-	-DMYSQL_UNIX_ADDR:STRING=/var/run/mysqld/mysqld.sock
+	-DMARIADB_UNIX_ADDR:STRING=/var/run/mysqld/mysqld.sock
 
 override_dh_makeshlibs:
 	dh_makeshlibs -X/mariadb3/plugin/