libubus: fix incompatible pointer types assigment

libubus: fix incompatible pointer types assigment

Fixes following error reported by clang-9 analyzer:

 libubus.c:286:19: error: incompatible pointer types assigning to 'struct blob_attr *' from 'char *' [-Werror,-Wincompatible-pointer-types]
         ctx->msgbuf.data = (char *) calloc(UBUS_MSG_CHUNK_SIZE, sizeof(char));

Result of 'calloc' is converted to a pointer of type 'struct blob_attr',
which is incompatible with sizeof operand type 'char'.

Signed-off-by: Petr Štetiar <ynezz@true.cz>