macos: fix missing pthread mutex init after calloc

calls constructor for a mutex in a struct value init-ed with gu_calloc.

in path gcs_core_create() -> gcs_group_init(), the first one allocates gcs_core_t* core with gu_calloc() whereas gcs_code_t has gcs_group_t group with gu::Mutex memb_mtx_. After memory allocation gu::Mutex constructor was not called that lead to an error on Darwin in a call to pthread mutex lock.

https://github.com/MariaDB/galera/pull/21