compilation warning in the ldb_mdb back end

[jhrozek]

../lib/ldb/ldb_mdb/ldb_mdb_ops.c: In function ‘ldb_mdb_add_op’: ../lib/ldb/ldb_mdb/ldb_mdb_ops.c:174:5: warning: passing argument 3 of ‘ldb_mdb_msg_store’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers] add_ctx->message,

../lib/ldb/ldb_mdb/ldb_mdb_util.h:66:5: note: expected ‘struct ldb_message ’ but argument is of type ‘const struct ldb_message ’ int ldb_mdb_msg_store(struct ldb_context *ldb,