ob1: add a destructor to mca_pml_ob1_recv_request_t

opal_mutex_t must be OBJ_DESTRUCTed in order to avoid
a memory leak (pthread_mutex_init allocates memory under
Cygwin, so pthread_mutex_destroy is mandatory)

Thanks to Marco Atzeri for reporting this issue
(cherry picked from commit open-mpi/ompi@ed93c8787d17b470bc7c1afe80536d8cacdd862d)